feat(foreground): foreground process sensors + translation key migration

Adds Home Assistant entities for the foreground-process feature shipped
in the media server, plus migrates existing display entities to use HA
translation keys (strings.json / translations/*) so per-language UI text
flows through the standard locale mechanism.

Foreground entities (all share one HA "Foreground" device linked to the
hub via via_device):
- sensor.foreground_process — process name as state + full payload
  (pid, exec path, window title, fullscreen flag, monitor, geometry,
  is_browser, browser_page_title, browser_url, error) as attributes
- sensor.window_title, sensor.pid, sensor.foreground_monitor,
  sensor.process_started (TIMESTAMP device class)
- binary_sensor.fullscreen, binary_sensor.minimized

Data flow:
- ForegroundCoordinator polls GET /api/foreground every 5s (HTTP fallback)
- media_player's WebSocket receiver forwards `foreground` /
  `foreground_update` push frames into the coordinator via
  apply_websocket_snapshot, so sensors update in near-real-time when WS
  is connected and fall back to polling otherwise

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

custom_components/remote_media_player/const.py

@@ -41,6 +41,7 @@ API_WEBSOCKET = "/api/media/ws"
 API_BROWSER_FOLDERS = "/api/browser/folders"
 API_BROWSER_BROWSE = "/api/browser/browse"
 API_BROWSER_PLAY = "/api/browser/play"
+API_FOREGROUND = "/api/foreground"
 API_DISPLAY_MONITORS = "/api/display/monitors"
 API_DISPLAY_BRIGHTNESS = "/api/display/brightness"
 API_DISPLAY_POWER = "/api/display/power"