feat(web): add i18n, light/dark theme switching, and version constant

- Add i18n (English + 简体中文) with a locale store, $t() magic, and a language toggle (persisted)
- Add light/dark/system theme switching via a theme store and [data-theme] token overrides
- Set the theme before paint to avoid a flash of the wrong theme
- Add frontend version constant web/version.js (window.YDROPBX_VERSION) shown in the app footer
- Update AGENTS.md version-control convention to mention web/version.js
- Bump version to 0.1.1 across frontend, webui changelog, and backend constant

internal/server/server.go

@@ -72,6 +72,7 @@ func (s *Server) Handler() http.Handler {
 	static := http.FileServerFS(web.FS)
 	mux.Handle("GET /components/", static)
 	mux.Handle("GET /styles/", static)
+	mux.Handle("GET /version.js", static)
 	mux.Handle("GET /alpine.min.js", static)
 
 	return mux