Add connection overlay and Gitea CI/CD workflow

Show full-screen overlay with spinner when server is unreachable,
with periodic health checks that auto-hide on reconnect.
Add Gitea Actions workflow for auto-deploy on release tags.

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

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

@@ -2,6 +2,8 @@
   "app.title": "LED Grab",
   "app.version": "版本：",
   "app.api_docs": "API 文档",
+  "app.connection_lost": "服务器不可达",
+  "app.connection_retrying": "正在尝试重新连接…",
   "theme.toggle": "切换主题",
   "accent.title": "主题色",
   "accent.custom": "自定义",