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

docs/CHANGELOG_webui.md

@@ -6,6 +6,22 @@ All notable changes to this template should be documented in this file.
 The format loosely follows Keep a Changelog and can be adapted to the team's habits.
 本文档参考了 Keep a Changelog 的思路，也可以根据团队习惯调整。
 
+## [0.1.1] - 2026-08-27
+
+### Added / 新增
+
+- Added internationalization (i18n): English + 简体中文, with a `locale` store, a `$t()` magic, and a language toggle persisted to localStorage.
+- 新增国际化（i18n）：支持 English 与简体中文，包含 `locale` store、`$t()` 魔法函数，以及持久化到 localStorage 的语言切换按钮。
+
+- Added light/dark/system theme switching via a `theme` store that writes `data-theme` on `<html>`; respects OS preference by default and persists the choice.
+- 新增浅色/深色/跟随系统主题切换：通过 `theme` store 在 `<html>` 上写入 `data-theme`，默认跟随系统并持久化用户选择。
+
+- Added dark-theme token overrides and a no-flash inline bootstrap script on both pages.
+- 为深色模式补充了设计令牌覆盖，并在两个页面加入防止主题闪烁的内联初始化脚本。
+
+- Added a frontend version constant (`web/version.js`, exposed as `window.YDROPBX_VERSION`) shown in the app footer, kept in sync with this changelog.
+- 新增前端版本常量（`web/version.js`，暴露为 `window.YDROPBX_VERSION`）并显示在应用页脚，与本文档版本保持一致。
+
 ## [0.1.0] - 2026-08-27
 
 ### Changed / 变更