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

py/services/settings_manager.py

@@ -994,6 +994,10 @@ class SettingsManager:
             self._save_settings()
             logger.info(f"Deleted setting: {key}")
 
+    def keys(self) -> Iterable[str]:
+        """Return all setting keys."""
+        return self.settings.keys()
+
     def _prepare_portable_switch(self, use_portable: bool) -> None:
         """Prepare switching the settings storage location."""