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/api_client.py

@@ -31,6 +31,7 @@ from .const import (
     API_BROWSER_BROWSE,
     API_BROWSER_PLAY,
     API_DISPLAY_MONITORS,
+    API_FOREGROUND,
     API_DISPLAY_BRIGHTNESS,
     API_DISPLAY_POWER,
     API_DISPLAY_CONTRAST,
@@ -420,6 +421,15 @@ class MediaServerClient:
             "POST", f"{API_DISPLAY_PICTURE_MODE}/{monitor_id}", {"code": code}
         )
 
+    async def get_foreground(self) -> dict[str, Any]:
+        """Get the foreground window/process snapshot.
+
+        Returns the structured payload described in the media server's
+        ``ForegroundInfo`` dataclass: process name, window title, fullscreen
+        flag, owning monitor, geometry, and process start time.
+        """
+        return await self._request("GET", API_FOREGROUND)
+
 
 class MediaServerWebSocket:
     """WebSocket client for real-time media status updates."""
@@ -432,6 +442,7 @@ class MediaServerWebSocket:
         on_status_update: Callable[[dict[str, Any]], None],
         on_disconnect: Callable[[], None] | None = None,
         on_scripts_changed: Callable[[], None] | None = None,
+        on_foreground_update: Callable[[dict[str, Any]], None] | None = None,
     ) -> None:
         """Initialize the WebSocket client.
 
@@ -442,6 +453,7 @@ class MediaServerWebSocket:
             on_status_update: Callback when status update received
             on_disconnect: Callback when connection lost
             on_scripts_changed: Callback when scripts have changed
+            on_foreground_update: Callback when foreground process changes
         """
         self._host = host
         self._port = int(port)
@@ -449,6 +461,7 @@ class MediaServerWebSocket:
         self._on_status_update = on_status_update
         self._on_disconnect = on_disconnect
         self._on_scripts_changed = on_scripts_changed
+        self._on_foreground_update = on_foreground_update
         # The server's WS endpoint accepts an unauthenticated connection when
         # api_tokens is empty (see media.py:websocket_endpoint), so we only
         # append ?token=... when one was configured.
@@ -537,6 +550,9 @@ class MediaServerWebSocket:
                         _LOGGER.info("Scripts changed notification received")
                         if self._on_scripts_changed:
                             self._on_scripts_changed()
+                    elif msg_type in ("foreground", "foreground_update"):
+                        if self._on_foreground_update:
+                            self._on_foreground_update(data.get("data", {}))
                     elif msg_type == "pong":
                         _LOGGER.debug("Received pong")