feat: remove default Spotify credentials, require user's own API key

- Remove hardcoded Spotify client ID/secret from Go backend
- Spotify now requires user to provide their own credentials
- Deezer remains free (no credentials required)
- Update UI to show 'Free' badge for Deezer, 'API Key' for Spotify
- Show warning card when Spotify selected without credentials
- Add hasSpotifyCredentials check to platform bridge

lib/services/platform_bridge.dart

@@ -331,7 +331,6 @@ class PlatformBridge {
   }
 
   /// Set custom Spotify API credentials
-  /// Pass empty strings to use default credentials
   static Future<void> setSpotifyCredentials(String clientId, String clientSecret) async {
     await _channel.invokeMethod('setSpotifyCredentials', {
       'client_id': clientId,
@@ -339,6 +338,13 @@ class PlatformBridge {
     });
   }
 
+  /// Check if Spotify credentials are configured
+  /// Returns true if credentials are available (custom or env vars)
+  static Future<bool> hasSpotifyCredentials() async {
+    final result = await _channel.invokeMethod('hasSpotifyCredentials');
+    return result as bool;
+  }
+
   /// Pre-warm track ID cache for album/playlist tracks
   /// This runs in background and returns immediately
   /// Speeds up subsequent downloads by caching ISRC → Track ID mappings