feat(downloads): detect lost SAF grant and offer folder re-selection

A persisted SAF tree grant can die silently (folder deleted or renamed,
SD card removed, app data reset, system revocation). Downloads then fail
with no recovery path beyond digging through Settings.

- Add isSafTreeAccessible bridge method: the grant must still be in the
  system's persisted permission list and the tree writable
- Show a warning banner in Files & Folders when the saved SAF folder is
  no longer accessible, with an inline re-select action
- In the download error dialog, replace Retry with a Re-select folder
  action when the failure is a lost folder grant (Android SAF or iOS
  bookmark); a successful re-pick retries the item automatically
- Share the folder-access error messages as constants so the UI can
  match on them reliably

lib/l10n/app_localizations.dart

@@ -5335,6 +5335,24 @@ abstract class AppLocalizations {
   /// **'Pick any folder, including SD card'**
   String get storageModeSafSubtitle;
 
+  /// Title of the warning banner shown in Files settings when the saved SAF folder grant is no longer valid
+  ///
+  /// In en, this message translates to:
+  /// **'Download folder access lost'**
+  String get downloadFolderAccessLostTitle;
+
+  /// Subtitle of the warning banner shown when the saved SAF folder grant is no longer valid
+  ///
+  /// In en, this message translates to:
+  /// **'Downloads will fail until you re-select the folder'**
+  String get downloadFolderAccessLostSubtitle;
+
+  /// Button that reopens the folder picker to restore download folder access
+  ///
+  /// In en, this message translates to:
+  /// **'Re-select folder'**
+  String get downloadFolderReselect;
+
   /// Description shown in filename format editor
   ///
   /// In en, this message translates to:

lib/l10n/app_localizations_ar.dart

@@ -3129,6 +3129,16 @@ class AppLocalizationsAr extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_de.dart

@@ -3169,6 +3169,16 @@ class AppLocalizationsDe extends AppLocalizations {
   String get storageModeSafSubtitle =>
       'Wähle einen beliebigen Ordner, inklusive SD-Karte';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_en.dart

@@ -3129,6 +3129,16 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_es.dart

@@ -3129,6 +3129,16 @@ class AppLocalizationsEs extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_fr.dart

@@ -3213,6 +3213,16 @@ class AppLocalizationsFr extends AppLocalizations {
   String get storageModeSafSubtitle =>
       'Choisissez n\'importe quel dossier, y compris la carte SD';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_hi.dart

@@ -3129,6 +3129,16 @@ class AppLocalizationsHi extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_id.dart

@@ -3112,6 +3112,16 @@ class AppLocalizationsId extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_ja.dart

@@ -3116,6 +3116,16 @@ class AppLocalizationsJa extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_ko.dart

@@ -3114,6 +3114,16 @@ class AppLocalizationsKo extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_nl.dart

@@ -3129,6 +3129,16 @@ class AppLocalizationsNl extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_pt.dart

@@ -3129,6 +3129,16 @@ class AppLocalizationsPt extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_ru.dart

@@ -3184,6 +3184,16 @@ class AppLocalizationsRu extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_tr.dart

@@ -3160,6 +3160,16 @@ class AppLocalizationsTr extends AppLocalizations {
   String get storageModeSafSubtitle =>
       'SD kart dahil herhangi bir klasörü seçin';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_uk.dart

@@ -3176,6 +3176,16 @@ class AppLocalizationsUk extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/app_localizations_zh.dart

@@ -3129,6 +3129,16 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get storageModeSafSubtitle => 'Pick any folder, including SD card';
 
+  @override
+  String get downloadFolderAccessLostTitle => 'Download folder access lost';
+
+  @override
+  String get downloadFolderAccessLostSubtitle =>
+      'Downloads will fail until you re-select the folder';
+
+  @override
+  String get downloadFolderReselect => 'Re-select folder';
+
   @override
   String downloadFilenameDescription(
     Object album,

lib/l10n/arb/app_en.arb

@@ -4110,6 +4110,18 @@
   "@storageModeSafSubtitle": {
     "description": "Subtitle for SAF storage mode"
   },
+  "downloadFolderAccessLostTitle": "Download folder access lost",
+  "@downloadFolderAccessLostTitle": {
+    "description": "Title of the warning banner shown in Files settings when the saved SAF folder grant is no longer valid"
+  },
+  "downloadFolderAccessLostSubtitle": "Downloads will fail until you re-select the folder",
+  "@downloadFolderAccessLostSubtitle": {
+    "description": "Subtitle of the warning banner shown when the saved SAF folder grant is no longer valid"
+  },
+  "downloadFolderReselect": "Re-select folder",
+  "@downloadFolderReselect": {
+    "description": "Button that reopens the folder picker to restore download folder access"
+  },
   "downloadFilenameDescription": "Use {artist}, {title}, {album}, {track}, {year}, {date}, {disc} as placeholders.",
   "@downloadFilenameDescription": {
     "description": "Description shown in filename format editor"