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"
 }

server/src/wled_controller/static/locales/ru.json

@@ -927,5 +927,21 @@
   "search.group.pp_templates": "Шаблоны постобработки",
   "search.group.pattern_templates": "Шаблоны паттернов",
   "search.group.audio": "Аудиоисточники",
-  "search.group.value": "Источники значений"
+  "search.group.value": "Источники значений",
+
+  "settings.title": "Настройки",
+  "settings.backup.label": "Резервное копирование",
+  "settings.backup.hint": "Скачать всю конфигурацию (устройства, цели, потоки, шаблоны, профили) в виде одного JSON-файла.",
+  "settings.backup.button": "Скачать резервную копию",
+  "settings.backup.success": "Резервная копия скачана",
+  "settings.backup.error": "Ошибка скачивания резервной копии",
+  "settings.restore.label": "Восстановление конфигурации",
+  "settings.restore.hint": "Загрузите ранее сохранённый файл резервной копии для замены всей конфигурации. Сервер перезапустится автоматически.",
+  "settings.restore.button": "Восстановить из копии",
+  "settings.restore.confirm": "Это заменит ВСЮ конфигурацию и перезапустит сервер. Вы уверены?",
+  "settings.restore.success": "Конфигурация восстановлена",
+  "settings.restore.error": "Ошибка восстановления",
+  "settings.restore.restarting": "Сервер перезапускается...",
+  "settings.restore.restart_timeout": "Сервер не отвечает. Обновите страницу вручную.",
+  "settings.button.close": "Закрыть"
 }

server/src/wled_controller/static/locales/zh.json

@@ -927,5 +927,21 @@
   "search.group.pp_templates": "后处理模板",
   "search.group.pattern_templates": "图案模板",
   "search.group.audio": "音频源",
-  "search.group.value": "值源"
+  "search.group.value": "值源",
+
+  "settings.title": "设置",
+  "settings.backup.label": "备份配置",
+  "settings.backup.hint": "将所有配置（设备、目标、流、模板、配置文件）下载为单个 JSON 文件。",
+  "settings.backup.button": "下载备份",
+  "settings.backup.success": "备份下载成功",
+  "settings.backup.error": "备份下载失败",
+  "settings.restore.label": "恢复配置",
+  "settings.restore.hint": "上传之前下载的备份文件以替换所有配置。服务器将自动重启。",
+  "settings.restore.button": "从备份恢复",
+  "settings.restore.confirm": "这将替换所有配置并重启服务器。确定继续吗？",
+  "settings.restore.success": "配置已恢复",
+  "settings.restore.error": "恢复失败",
+  "settings.restore.restarting": "服务器正在重启...",
+  "settings.restore.restart_timeout": "服务器未响应。请手动刷新页面。",
+  "settings.button.close": "关闭"
 }