refactor: make subsync manual-only, default opt-in features off, preserv

- Remove subsync.defaultMode; subsync always opens manual picker
- Default jellyfinRemoteSession warmup and nameMatchEnabled to false
- Stop rewriting config file during legacy migration (resolve in-memory only)
- Fix macOS quit on window-close for --setup launch mode

src/config/service.ts

@@ -149,7 +149,6 @@ export class ConfigService {
       if (!migrated) {
         return rawConfig;
       }
-      fs.writeFileSync(configPath, content, 'utf-8');
       return rawConfig;
     } catch (error) {
       console.error(`[ConfigService] legacy config migration failed for ${configPath}:`, error);