feat: shared DisplayCoordinator + optional API token

- Introduce DisplayCoordinator polling /api/display/monitors once per
  cycle and fan out to all per-display entities via CoordinatorEntity.
  Removes ~9x redundant requests per polling cycle that came from each
  binary_sensor/number/select/sensor/switch entity calling
  get_display_monitors() in its own async_update.
- Optimistic write-through via coordinator.apply_optimistic(...) keeps
  sibling entities in sync after slider/select writes without an extra
  network round-trip.
- Make CONF_TOKEN optional. The media server already supports running
  without auth (auth_enabled() returns False when api_tokens is empty),
  so the integration omits the Authorization header and ?token= query
  from REST/WS/album-art URLs when no token is configured. Server-side
  auth-enabled rejections still surface as invalid_auth in the UI.
- Bump manifest version to 0.3.2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

custom_components/remote_media_player/const.py

@@ -16,6 +16,10 @@ DEFAULT_POLL_INTERVAL = 5
 DEFAULT_NAME = "Remote Media Player"
 DEFAULT_USE_WEBSOCKET = True
 DEFAULT_RECONNECT_INTERVAL = 5
+# Displays change rarely (brightness/contrast/input source via physical buttons
+# or external automations), so a slow shared poll is plenty. The previous
+# per-entity polling produced ~9 calls every 30 s for a single monitor.
+DEFAULT_DISPLAY_POLL_INTERVAL = 30
 
 # API endpoints
 API_HEALTH = "/api/health"