v0.13.0 — detached notepad window

Detach the notepad into an independent window that survives the DSH interface being minimized or closed.

• A new header button and a Settings → Notepad → Detached window card open the same workspace note in a plugin-owned standalone window.
• The DSH floating panel collapses to its restore pill while detached; closing the standalone window, pressing its "Return to panel" button, the restore pill, or the Settings action brings the panel back.
• The standalone page is served by a token-guarded loopback server, not the DSH login route, so an external browser window keeps reading and writing <workspace>/.dsh/notepad.json while DSH is minimized or its page is closed.
• The detached window has its own minimize / restore, editing, Enter split, Backspace empty-card removal, archive / restore / delete, copy, AI improvement through the Host model selection, image preview, auto / manual save, dark mode and the configured theme and typography.
• Heartbeats and page-exit beacons keep the panel and window from fighting over the same note; a retired window is not revived by a URL prefetch.
• Security: the window token is random per Host process, the server binds 127.0.0.1 only, every API requires the token and a same-origin request, and images are confined to the workspace notepad image directory.

独立窗口：让记事本脱离 DSH 界面，DSH 最小化或关闭页面时仍可显示和编辑。

• 标题栏新增「↗」按钮，设置页新增「独立窗口」卡片；脱离期间浮窗收起为右下角小圆钮。
• 独立窗口有自己的最小化 / 还原，支持编辑、Enter 拆分、段首 Backspace 删除空段、归档 / 恢复 / 删除、复制、AI 优化、图片预览、自动 / 手动保存和深色模式。
• 独立窗口由插件自己的回环服务 + 随机 token 提供，不经过 DSH 登录 Cookie，所以在 DSH 最小化或页面关闭后仍能继续写回同一份工作区笔记；DSH 浮窗会通过心跳检测窗口关闭并自动恢复。
• 安全：只监听 127.0.0.1，所有 API 要求 token + 同源，图片只能读取该工作区 .dsh/notepad/images/。