fix(browser): auto-reveal session tabs and prevent preview clipping

[devswha]

Scope

Focused browser-panel follow-up stacked on #49. This PR contains only the browser auto-reveal/layout work; merge #49 first and retarget this PR to main afterward. It does not rebuild, install, or replace published beta.10 assets.

Problem and behavior

• The preview only listened for browser activity after the user manually mounted the Browser tab. Watch validated, current-session metadata while the panel is closed, revealing its first active tab and newly opened tabs. Routine state updates/reconnects respect manual dismissal and tab selection.
• The watcher reads the existing sidecar recovery cache. It does not launch Chromium, subscribe to image frames, create grants, or stop a visible preview when disconnected. Browser session types and validation now have one shared definition.
• Saved rail widths were only clamped while dragging, so a smaller window/sidebar could clip the preview and toolbar. Re-clamp on container attachment/resize and preserve chat's 200px minimum.
• Keep images inside the measured surface; sync Chromium after status-before-mount races and reconnects; map clicks against the loaded frame instead of a pending requested viewport. Hide mismatched previous-tab images.

Reproduction and visual evidence

Restore a 1200px browser rail, then use a 1024×768 window. The old controller placed its right edge at x=1400; the new one keeps it at x=1024.

The comparison is an isolated fixture using the real WorkspacePanel/BrowserPanel, app CSS, React Compiler and Chromium sidecar. The before fixture replays the width controller from 1339b672, with the same viewer/test page. See docs/BROWSER-CUA-VERIFICATION.md and the checked-in before/after images under docs/images/browser-panel/.

Before:

After:

Verification

• npm run verify passed (audit policy, TypeScript, Rust core, all tests, lint/boundaries, identity, production builds).
• npm run test:e2e:browser: 3 passed.
• npm run test:e2e:gjc: 8 passed.
• New DOM/server regressions cover idle metadata watching, first-tab reveal, manual dismissal, reconnects, foreign/retired sessions, delayed status, loaded-frame click mapping, frame cleanup and restored desktop widths.
• Interactive Browser-skill QA: 1024×768, 768×768, 390×844, 320×568 and expanded 1600×1200. All page corners visible; real bottom-right input reached the page button. Initial agent-open and new-tab auto-reveal worked; ordinary navigation stayed dismissed.
• No packaged Tauri GUI, release installer, or installed-app update is claimed. Current installed beta.10 remains unchanged.