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

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -2414,6 +2414,27 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(exists)
                         }
+                        "isSafTreeAccessible" -> {
+                            val uriStr = call.argument<String>("tree_uri") ?: ""
+                            val accessible = withContext(Dispatchers.IO) {
+                                try {
+                                    val uri = Uri.parse(uriStr)
+                                    val persisted = contentResolver.persistedUriPermissions.any {
+                                        it.uri == uri && it.isReadPermission && it.isWritePermission
+                                    }
+                                    if (!persisted) {
+                                        false
+                                    } else {
+                                        val doc = DocumentFile.fromTreeUri(this@MainActivity, uri)
+                                        doc != null && doc.exists() && doc.canWrite()
+                                    }
+                                } catch (e: Exception) {
+                                    android.util.Log.w("SpotiFLAC", "SAF tree access check failed: ${e.message}")
+                                    false
+                                }
+                            }
+                            result.success(accessible)
+                        }
                         "safDelete" -> {
                             val uriStr = call.argument<String>("uri") ?: ""
                             val deleted = withContext(Dispatchers.IO) {