fix(web-ui): set the browser tab title from the open conversation

[dannyyol]

Summary

• Set the Web UI browser tab title from the open conversation (e.g. Choose assistant tone · QM) instead of a fixed QM · Web for every chat
• Keep it in sync when switching conversations, renaming, auto-title refresh, and changing focused split panes
• Fall back to existing untitled labels (New chat, Web chat, …) or QM · Web when nothing is open

Fixes #345

Test plan

• Open two conversations in separate tabs — each tab title should show that conversation’s name
• Rename a conversation — tab title updates without a reload
• Start a new untitled chat — tab shows a sensible fallback, then the generated title once it arrives
• Switch to a non-chat view (e.g. Files) — title returns to QM · Web (or branded equivalent)
• In split view, focus different panes — tab title follows the focused pane
• cd plugins/web-ui && npm test -- test/document-title.test.ts
• cd plugins/web-ui && npm run typecheck

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}