refactor: remove built-in provider registry in favor of extensions

All search, metadata, and download providers are now exclusively
supplied by extensions. The built-in provider registry that previously
exposed Deezer/Tidal/Qobuz as hardcoded providers is fully removed.

Removed across Go, Dart, Kotlin, and Swift:
- BuiltInProviderSpec class, registry, and all accessor helpers
- SearchProviderAllJSON, GetBuiltInProvidersJSON, ParseProviderURLJSON,
  ParseDeezerURLExport Go exports and their platform channel bindings
- Built-in provider items in search dropdown, service picker, and
  provider priority UI lists
- provider_ui_utils.dart helper file

Deezer metadata enrichment (ISRC lookup, extended metadata, cover
upgrade) remains fully functional through direct DeezerClient calls
in the download pipeline — these are not part of the provider
registry and are unaffected.

Mark deezer as a retired built-in metadata provider so stale user
priority lists are cleaned up on next launch.

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

@@ -2795,23 +2795,6 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(null)
                         }
-                        "searchProviderAll" -> {
-                            val providerId = call.argument<String>("provider_id") ?: ""
-                            val query = call.argument<String>("query") ?: ""
-                            val trackLimit = call.argument<Int>("track_limit") ?: 15
-                            val artistLimit = call.argument<Int>("artist_limit") ?: 2
-                            val filter = call.argument<String>("filter") ?: ""
-                            val response = withContext(Dispatchers.IO) {
-                                Gobackend.searchProviderAllJSON(providerId, query, trackLimit.toLong(), artistLimit.toLong(), filter)
-                            }
-                            result.success(response)
-                        }
-                        "getBuiltInProviders" -> {
-                            val response = withContext(Dispatchers.IO) {
-                                Gobackend.getBuiltInProvidersJSON()
-                            }
-                            result.success(response)
-                        }
                         "getDeezerRelatedArtists" -> {
                             val artistId = call.argument<String>("artist_id") ?: ""
                             val limit = call.argument<Int>("limit") ?: 12
@@ -2829,13 +2812,6 @@ class MainActivity: FlutterFragmentActivity() {
                             }
                             result.success(response)
                         }
-                        "parseProviderUrl" -> {
-                            val url = call.argument<String>("url") ?: ""
-                            val response = withContext(Dispatchers.IO) {
-                                Gobackend.parseProviderURLJSON(url)
-                            }
-                            result.success(response)
-                        }
                         "searchDeezerByISRC" -> {
                             val isrc = call.argument<String>("isrc") ?: ""
                             val itemId = call.argument<String>("item_id") ?: ""

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

@@ -205,6 +205,7 @@ object SafDownloadHandler {
         mimeType: String,
         srcPath: String
     ): String? {
+        var stagedDocument: DocumentFile? = null
         return try {
             val treeUri = Uri.parse(treeUriStr)
             val targetDir = ensureDocumentDir(context, treeUri, relativeDir) ?: return null
@@ -213,11 +214,18 @@ object SafDownloadHandler {
             val stagedName = buildStagedSafFileName(finalName, ext)
             val document = createOrReuseDocumentFile(targetDir, mimeType, stagedName)
                 ?: return null
-            context.contentResolver.openOutputStream(document.uri, "wt")?.use { output ->
+            stagedDocument = document
+            val outputStream = context.contentResolver.openOutputStream(document.uri, "wt")
+            if (outputStream == null) {
+                document.delete()
+                stagedDocument = null
+                return null
+            }
+            outputStream.use { output ->
                 File(srcPath).inputStream().use { input ->
                     input.copyTo(output)
                 }
-            } ?: return null
+            }
 
             val existingFinal = targetDir.findFile(finalName)
             if (existingFinal != null && existingFinal.uri != document.uri) {
@@ -227,8 +235,10 @@ object SafDownloadHandler {
                 document.delete()
                 return null
             }
+            stagedDocument = null
             targetDir.findFile(finalName)?.uri?.toString() ?: document.uri.toString()
         } catch (e: Exception) {
+            stagedDocument?.delete()
             android.util.Log.w("SpotiFLAC", "Failed to write file to SAF: ${e.message}")
             null
         }