fix(web): show mobile doc selector when no KB doc is selected

[contextmatrix-runner]

Summary

After CTXMAX-476 made the Knowledge Base tab mobile-responsive, mobile users opening the KB tab with no doc selected saw "Select a doc" with no way to open the doc selector — the desktop sidebar is hidden on mobile (hidden md:flex) and the only sheet trigger lived inside KnowledgeDocViewer / KnowledgeDocEditor, which only render when a doc IS selected. Same dead-end for the "No knowledge base docs yet." fallback when the project has no built KB — the Refresh button is in the unreachable sidebar.

Changes

• New MobileDocTrigger component (open-book icon + label + chevron, md:hidden).
• Render the trigger in both no-doc-selected fallback branches in KnowledgeBase.tsx so mobile users can always reach the MobileDocSheet.
• Refactor KnowledgeDocViewer and KnowledgeDocEditor to consume the shared component (dedupes ~20 lines of SVG markup per call site, drops both files in line with the ~150-line guideline).
• Update web/CLAUDE.md Mobile Knowledge Base architecture table and Behaviour subsection.

Test plan

• cd web && npm run lint clean
• cd web && npm run build clean
• Manual smoke on a mobile-width viewport (Chrome devtools, iPhone 15 preset, 393×852):
  • KB tab on a project with built docs and no doc selected → "Choose a document" trigger → tap → sheet opens → tap a doc → sheet closes → trigger now shows the doc name
  • KB tab on a project with no built KB → trigger above "No knowledge base docs yet." → tap → sheet opens with Refresh button reachable
  • Resize ≥768px → trigger hidden, desktop two-column layout unchanged