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/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"
 }