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/en.json

@@ -2,6 +2,8 @@
   "app.title": "LED Grab",
   "app.version": "Version:",
   "app.api_docs": "API Documentation",
+  "app.connection_lost": "Server unreachable",
+  "app.connection_retrying": "Attempting to reconnect…",
   "theme.toggle": "Toggle theme",
   "accent.title": "Accent color",
   "accent.custom": "Custom",