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>

docs/API.md

@@ -42,7 +42,7 @@ Profiles are selected with **`POST /profiles/<id>/apply`**, which sets `current_
 | Method | Path | Description |
 |--------|------|-------------|
 | GET | `/` | Main UI (`templates/index.html`) |
-| GET | `/settings` | Settings page (`templates/settings.html`) |
+| GET | `/settings/page` | Standalone settings page (`templates/settings.html`) |
 | GET | `/favicon.ico` | Empty response (204) |
 | GET | `/static/<path>` | Static files under `src/static/` |
 
@@ -72,7 +72,7 @@ Below, `<id>` values are string identifiers used by the JSON stores (numeric str
 | PUT | `/settings/settings` | Merge keys into settings and save. Returns `{"message": "Settings updated successfully"}`. |
 | GET | `/settings/wifi/ap` | Saved Wi‑Fi AP fields: `saved_ssid`, `saved_password`, `saved_channel`, `active` (Pi: `active` is always false). |
 | POST | `/settings/wifi/ap` | Body: `ssid` (required), `password`, `channel` (1–11). Persists AP-related settings. |
-| GET | `/settings/page` | Serves `templates/settings.html` (same page as `GET /settings` from the root app, for convenience). |
+| GET | `/settings/page` | Serves `templates/settings.html`. |
 
 ### Devices — `/devices`