refactor(settings): invert sync logic from whitelist to blacklist

Replace _SYNC_KEYS (37 keys) with _NO_SYNC_KEYS (5 keys) in SettingsHandler.
New settings automatically sync to frontend unless explicitly excluded.

Changes:
- SettingsHandler now syncs all settings except those in _NO_SYNC_KEYS
- Added keys() method to SettingsManager for iteration
- Updated tests to use new behavior

Benefits:
- No more missing keys when adding new settings
- Reduced maintenance burden
- Explicit exclusions for sensitive/internal settings only

Fixes: #86

tests/routes/__snapshots__/test_api_snapshots.ambr

@@ -26,6 +26,7 @@
     'settings': dict({
       'civitai_api_key': 'test-key',
       'language': 'en',
+      'theme': 'dark',
     }),
     'success': True,
   })