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/js/app.js

@@ -74,6 +74,10 @@ import {
     toggleDynamicBackground, applyDynamicBackground, updateBackgroundColors,
 } from './background.js';
 
+import {
+    updateForegroundUI, loadForegroundProcess,
+} from './foreground.js';
+
 // ============================================================
 // Register late-bound callbacks for core's updateAllText()
 // ============================================================
@@ -136,6 +140,8 @@ Object.assign(window, {
     onAudioDeviceChanged,
     // About
     showAboutDialog, closeAboutDialog,
+    // Foreground
+    loadForegroundProcess,
 });
 
 // ============================================================