feat(settings): server global brightness and Wi-Fi driver resync

- Serve GET /settings as JSON by removing duplicate HTML route (use /settings/page for the standalone UI).

- Save global_brightness via PUT; broadcast to connected drivers; push saved level when outbound WS connects.

- Zones UI loads brightness from GET /settings only (no localStorage).

- Bump led-driver submodule for settings.save on brightness with save flag.

- Extend API doc and endpoint tests for global_brightness.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/settings.py

@@ -73,6 +73,9 @@ class Settings(dict):
         # UART to ESP32 ESP-NOW bridge; default off (Wi-Fi drivers need no serial).
         if 'serial_enabled' not in self:
             self['serial_enabled'] = False
+        # Zone UI global brightness (0–255); shared across browsers/devices.
+        if 'global_brightness' not in self:
+            self['global_brightness'] = 255
 
     def save(self):
         try: