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/static/js/features/settings.ts

@@ -347,9 +347,6 @@ export async function handleRestoreFileSelected(input: HTMLInputElement): Promis
         showToast(data.message || t('settings.restore.success'), 'success');
         settingsModal.forceClose();
 
-        if (data.restart_scheduled) {
-            showRestartOverlay();
-        }
     } catch (err) {
         console.error('Restore failed:', err);
         showToast(t('settings.restore.error') + ': ' + err.message, 'error');
@@ -369,63 +366,12 @@ export async function restartServer(): Promise<void> {
             throw new Error(err.detail || `HTTP ${resp.status}`);
         }
         settingsModal.forceClose();
-        showRestartOverlay(t('settings.restarting'));
     } catch (err) {
         console.error('Server restart failed:', err);
         showToast(t('settings.restore.error') + ': ' + err.message, 'error');
     }
 }
 
-// ─── Restart overlay ───────────────────────────────────────
-
-function showRestartOverlay(message?: string): void {
-    const msg = message || t('settings.restore.restarting');
-    const overlay = document.createElement('div');
-    overlay.id = 'restart-overlay';
-    overlay.style.cssText =
-        'position:fixed;inset:0;z-index:100000;display:flex;flex-direction:column;' +
-        'align-items:center;justify-content:center;background:rgba(0,0,0,0.85);color:#fff;font-size:1.2rem;';
-    overlay.innerHTML =
-        '<div class="spinner" style="width:48px;height:48px;border:4px solid rgba(255,255,255,0.3);' +
-        'border-top-color:#fff;border-radius:50%;animation:spin 0.8s linear infinite;margin-bottom:1rem;"></div>' +
-        `<div id="restart-msg">${msg}</div>`;
-
-    // Add spinner animation if not present
-    if (!document.getElementById('restart-spinner-style')) {
-        const style = document.createElement('style');
-        style.id = 'restart-spinner-style';
-        style.textContent = '@keyframes spin{to{transform:rotate(360deg)}}';
-        document.head.appendChild(style);
-    }
-
-    document.body.appendChild(overlay);
-    pollHealth();
-}
-
-function pollHealth(): void {
-    const start = Date.now();
-    const maxWait = 30000;
-    const interval = 1500;
-
-    const check = async () => {
-        if (Date.now() - start > maxWait) {
-            const msg = document.getElementById('restart-msg');
-            if (msg) msg.textContent = t('settings.restore.restart_timeout');
-            return;
-        }
-        try {
-            const resp = await fetch('/health', { signal: AbortSignal.timeout(3000) });
-            if (resp.ok) {
-                window.location.reload();
-                return;
-            }
-        } catch { /* server still down */ }
-        setTimeout(check, interval);
-    };
-    // Wait a moment before first check to let the server shut down
-    setTimeout(check, 2000);
-}
-
 // ─── Auto-Backup settings ─────────────────────────────────
 
 export async function loadAutoBackupSettings(): Promise<void> {
@@ -567,9 +513,6 @@ export async function restoreSavedBackup(filename: string): Promise<void> {
         showToast(data.message || t('settings.restore.success'), 'success');
         settingsModal.forceClose();
 
-        if (data.restart_scheduled) {
-            showRestartOverlay();
-        }
     } catch (err) {
         console.error('Restore from saved backup failed:', err);
         showToast(t('settings.restore.error') + ': ' + err.message, 'error');
@@ -687,9 +630,6 @@ export async function handlePartialImportFileSelected(input: HTMLInputElement):
         showToast(data.message || t('settings.partial.import_success'), 'success');
         settingsModal.forceClose();
 
-        if (data.restart_scheduled) {
-            showRestartOverlay();
-        }
     } catch (err) {
         console.error('Partial import failed:', err);
         showToast(t('settings.partial.import_error') + ': ' + err.message, 'error');