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/components/layout/Header.tsx

@@ -1960,7 +1960,7 @@ export const Header: React.FC<HeaderProps> = ({
         onClick={toggleBottomTerminal}
         Icon={'terminal-box'}
       />
-      {hasElectronDesktopIPC ? (
+      {!isMobile ? (
         <HeaderIconActionButton
           title={t('contextPanel.browser.open')}
           ariaLabel={t('contextPanel.browser.open')}