feat: browser-side annotation screenshots for web preview

Capture the preview/browser iframe DOM with snapDOM (html-to-image
fallback) so web annotation screenshots match the visible viewport,
without a headless Chromium dependency.

- Preserve document scroll via viewport crop and re-bake nested scroll
  (e.g. the Starlight sidebar) deterministically on the clone
- Pin position:fixed elements to their measured viewport rect so headers
  and sidebars land correctly in the crop
- Extract preview capture/proxy helpers into
  lib/preview/screenshot-capture.ts to slim down ContextPanel
- Guard the external preview proxy against SSRF to private, loopback and
  reserved/link-local addresses (incl. cloud metadata)
- Fully validate preview bridge messages before formatting/use
- Warn on the empty browser tab that pages run with full access, so
  users browse untrusted sites knowingly

packages/ui/src/lib/i18n/messages/zh-CN.ts

@@ -843,6 +843,8 @@ export const dict: Record<I18nKey, string> = {
   'contextPanel.browser.addressAria': '浏览器地址',
   'contextPanel.browser.empty': '网页浏览器',
   'contextPanel.browser.emptyHint': '在上方输入网址开始浏览',
+  'contextPanel.browser.inspectUnavailable': '无法从浏览器面板检查此页面。',
+  'contextPanel.browser.trustNotice': '在此打开的页面以对 OpenChamber 的完全访问权限运行 — 检查和截图需要此权限。仅打开你信任的站点：恶意页面可能读取你的数据或以你的身份执行操作。',
   'contextPanel.tab.closeTabAria': '关闭 {label} 标签',
   'contextPanel.actions.collapsePanel': '折叠面板',
   'contextPanel.actions.expandPanel': '展开面板',