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/locales/en.json

@@ -927,5 +927,21 @@
   "search.group.pp_templates": "Post-Processing Templates",
   "search.group.pattern_templates": "Pattern Templates",
   "search.group.audio": "Audio Sources",
-  "search.group.value": "Value Sources"
+  "search.group.value": "Value Sources",
+
+  "settings.title": "Settings",
+  "settings.backup.label": "Backup Configuration",
+  "settings.backup.hint": "Download all configuration (devices, targets, streams, templates, profiles) as a single JSON file.",
+  "settings.backup.button": "Download Backup",
+  "settings.backup.success": "Backup downloaded successfully",
+  "settings.backup.error": "Backup download failed",
+  "settings.restore.label": "Restore Configuration",
+  "settings.restore.hint": "Upload a previously downloaded backup file to replace all configuration. The server will restart automatically.",
+  "settings.restore.button": "Restore from Backup",
+  "settings.restore.confirm": "This will replace ALL configuration and restart the server. Are you sure?",
+  "settings.restore.success": "Configuration restored",
+  "settings.restore.error": "Restore failed",
+  "settings.restore.restarting": "Server is restarting...",
+  "settings.restore.restart_timeout": "Server did not respond. Please refresh the page manually.",
+  "settings.button.close": "Close"
 }