feat: graceful shutdown with store persistence and restart overlay

- Add /api/v1/system/shutdown endpoint that triggers clean uvicorn exit
- Persist all 15 stores to disk during shutdown via _save_all_stores()
- Add force parameter to BaseJsonStore._save() to bypass restore freeze
- Restart script now requests graceful shutdown via API (15s timeout),
  falls back to force-kill only if server doesn't exit in time
- Broadcast server_restarting event over WebSocket before shutdown
- Frontend shows "Server restarting..." overlay instantly on WS event,
  replacing the old dynamically-created overlay from settings.ts
- Add server_ref module to share uvicorn Server + TrayManager refs
- Add i18n keys for restart overlay (en/ru/zh)

server/src/wled_controller/templates/index.html

@@ -25,8 +25,14 @@
         <div id="connection-overlay" class="connection-overlay" style="display:none" aria-hidden="true">
             <div class="connection-overlay-content">
                 <div class="connection-spinner-lg"></div>
-                <h2 data-i18n="app.connection_lost">Server unreachable</h2>
-                <p data-i18n="app.connection_retrying">Attempting to reconnect…</p>
+                <div id="conn-msg-offline">
+                    <h2 data-i18n="app.connection_lost">Server unreachable</h2>
+                    <p data-i18n="app.connection_retrying">Attempting to reconnect…</p>
+                </div>
+                <div id="conn-msg-restarting" style="display:none">
+                    <h2 data-i18n="app.server_restarting">Server restarting…</h2>
+                    <p data-i18n="app.server_restarting_sub">Please wait, the server will be back shortly.</p>
+                </div>
             </div>
         </div>
         <header>