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/index.html

@@ -535,7 +535,7 @@
             </details>
         </div>
 
-        <!-- Display Control Section -->
+        <!-- Display Control Section (monitors first, foreground overview below) -->
         <div class="display-container" data-tab-content="display" role="tabpanel" id="panel-display">
             <div class="display-monitors" id="displayMonitors">
                 <div class="empty-state-illustration">
@@ -543,6 +543,12 @@
                     <p data-i18n="display.loading">Loading monitors...</p>
                 </div>
             </div>
+            <div class="foreground-stage" id="foregroundStage">
+                <div class="empty-state-illustration">
+                    <svg viewBox="0 0 64 64"><rect x="6" y="12" width="52" height="36" rx="3" fill="none" stroke="currentColor" stroke-width="2"/><line x1="6" y1="20" x2="58" y2="20" stroke="currentColor" stroke-width="2"/><circle cx="11" cy="16" r="1.4" fill="currentColor"/><circle cx="15" cy="16" r="1.4" fill="currentColor"/><circle cx="19" cy="16" r="1.4" fill="currentColor"/></svg>
+                    <p data-i18n="foreground.loading">Waiting for foreground signal…</p>
+                </div>
+            </div>
         </div>
     </div>