feat(foreground): track topmost process + browser page title

Adds cross-platform foreground-window tracking and exposes it over REST
(/api/foreground) and the existing WebSocket feed.

- foreground_service.py: Windows probe via ctypes (HANDLE-correct argtypes
  to avoid 64-bit handle truncation); macOS via AppKit; Linux via Xlib
  (Wayland returns unavailable). TTL cache + per-platform fallback.
- browser_url_service.py: when foreground is a recognised browser, extract
  the page title from the window title (browser-name suffix stripped) and
  surface `is_browser` + `browser_page_title`. Optional UIA-based URL
  extraction behind MEDIA_SERVER_BROWSER_UIA env flag (off by default —
  Chromium browsers keep their accessibility tree dormant otherwise).
- websocket_manager: poll foreground every 1s inside the existing status
  loop, broadcast `foreground` on connect and `foreground_update` on
  change. Diff only on user-visible fields to avoid geometry spam.
- WebUI: new editorial card rendered under the monitor list on the
  Display tab — process name, window title, fullscreen/minimized/monitor
  chips, browser block when applicable, exe path, PID, started-ago,
  geometry, platform. 16px inter-section gap matches Settings cadence.
- i18n: 25 new keys added to both en.json and ru.json.

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

media_server/static/locales/ru.json

@@ -292,5 +292,29 @@
   "about.source_code": "Исходный код",
   "dialog.close": "Закрыть",
   "update.available": "Доступно обновление: v{version}",
-  "update.view_release": "Перейти к релизу"
+  "update.view_release": "Перейти к релизу",
+  "tab.foreground": "Активное окно",
+  "foreground.kicker": "Активное окно",
+  "foreground.loading": "Ожидание сигнала об активном окне…",
+  "foreground.no_process": "Активное окно не определено",
+  "foreground.unavailable": "Отслеживание активного окна недоступно",
+  "foreground.process": "Процесс",
+  "foreground.window_title": "Заголовок окна",
+  "foreground.executable": "Путь к программе",
+  "foreground.pid": "PID",
+  "foreground.monitor": "Монитор {n}",
+  "foreground.started": "Запущено",
+  "foreground.geometry": "Геометрия",
+  "foreground.platform": "Платформа",
+  "foreground.fullscreen": "Полноэкранный",
+  "foreground.minimized": "Свёрнут",
+  "foreground.windowed": "Оконный",
+  "foreground.browser": "Браузер",
+  "foreground.page_title": "Заголовок страницы",
+  "foreground.url": "URL",
+  "foreground.badge.title": "Открыть активное окно",
+  "foreground.ago.seconds": "{n} с назад",
+  "foreground.ago.minutes": "{n} мин назад",
+  "foreground.ago.hours": "{n} ч {m} мин назад",
+  "foreground.ago.days": "{n} дн назад"
 }