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/strings.json

@@ -14,7 +14,7 @@
         "data_description": {
           "host": "Hostname or IP address of the Media Server",
           "port": "Port number (default: 8765)",
-          "token": "API authentication token from the server configuration",
+          "token": "API authentication token from the server configuration. Leave blank if the server runs without authentication.",
           "name": "Display name for this media player",
           "poll_interval": "How often to poll for status updates (seconds)"
         }