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.

ios/Runner/AppDelegate.swift

@@ -458,22 +458,6 @@ import Gobackend  // Import Go framework
             if let error = error { throw error }
             return response
             
-        case "searchProviderAll":
-            let args = call.arguments as! [String: Any]
-            let providerId = args["provider_id"] as! String
-            let query = args["query"] as! String
-            let trackLimit = args["track_limit"] as? Int ?? 15
-            let artistLimit = args["artist_limit"] as? Int ?? 3
-            let filter = args["filter"] as? String ?? ""
-            let response = GobackendSearchProviderAllJSON(providerId, query, Int(trackLimit), Int(artistLimit), filter, &error)
-            if let error = error { throw error }
-            return response
-
-        case "getBuiltInProviders":
-            let response = GobackendGetBuiltInProvidersJSON(&error)
-            if let error = error { throw error }
-            return response
-
         case "getDeezerRelatedArtists":
             let args = call.arguments as! [String: Any]
             let artistId = args["artist_id"] as! String
@@ -491,13 +475,6 @@ import Gobackend  // Import Go framework
             if let error = error { throw error }
             return response
 
-        case "parseProviderUrl":
-            let args = call.arguments as! [String: Any]
-            let url = args["url"] as! String
-            let response = GobackendParseProviderURLJSON(url, &error)
-            if let error = error { throw error }
-            return response
-
         case "searchDeezerByISRC":
             let args = call.arguments as! [String: Any]
             let isrc = args["isrc"] as! String