Add configuration backup/restore with settings modal

Backend: GET /api/v1/system/backup bundles all 11 store JSON files into a
single downloadable backup with metadata envelope. POST /api/v1/system/restore
validates and writes stores atomically, then schedules a delayed server restart
via detached restart.ps1 subprocess.

Frontend: Settings modal (gear button in header) with Download Backup and
Restore from Backup buttons. Restore shows confirm dialog, uploads via
multipart FormData, then displays fullscreen restart overlay that polls
/health until the server comes back and reloads the page.

Locales: en, ru, zh translations for all settings.* keys.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/js/app.js

@@ -131,10 +131,13 @@ import {
     showCSSCalibration, toggleCalibrationOverlay,
 } from './features/calibration.js';
 
-// Layer 6: tabs, navigation, command palette
+// Layer 6: tabs, navigation, command palette, settings
 import { switchTab, initTabs, startAutoRefresh, handlePopState } from './features/tabs.js';
 import { navigateToCard } from './core/navigation.js';
 import { openCommandPalette, closeCommandPalette, initCommandPalette } from './core/command-palette.js';
+import {
+    openSettingsModal, closeSettingsModal, downloadBackup, handleRestoreFileSelected,
+} from './features/settings.js';
 
 // ─── Register all HTML onclick / onchange / onfocus globals ───
 
@@ -384,6 +387,12 @@ Object.assign(window, {
     navigateToCard,
     openCommandPalette,
     closeCommandPalette,
+
+    // settings (backup / restore)
+    openSettingsModal,
+    closeSettingsModal,
+    downloadBackup,
+    handleRestoreFileSelected,
 });
 
 // ─── Global keyboard shortcuts ───