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

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

@@ -218,6 +218,12 @@ class MainActivity: FlutterActivity() {
                             }
                             result.success(null)
                         }
+                        "hasSpotifyCredentials" -> {
+                            val hasCredentials = withContext(Dispatchers.IO) {
+                                Gobackend.checkSpotifyCredentials()
+                            }
+                            result.success(hasCredentials)
+                        }
                         "preWarmTrackCache" -> {
                             val tracksJson = call.argument<String>("tracks") ?: "[]"
                             withContext(Dispatchers.IO) {