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

web/web.go

@@ -2,5 +2,5 @@ package web
 
 import "embed"
 
-//go:embed login.html index.html alpine.min.js components styles
+//go:embed login.html index.html alpine.min.js version.js components styles
 var FS embed.FS