feat: add PSA ticket export format and Quick-Start landing page

PSA Export:
- New "PSA / Ticket Note" export format optimized for ConnectWise
- Structured output: Problem, Steps Taken, Resolution, Time Spent, Notes
- Prominent "Copy for Ticket" button on session detail page
- 24 unit tests for PSA export generator

Quick-Start Landing:
- New default landing page with search-first UX
- Auto-focused search bar with debounced tree search
- "Continue Session" cards for active sessions
- "Recent Trees" section from session history
- Home nav item and logo links updated

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/session/ExportPreviewModal.tsx

@@ -8,7 +8,7 @@ interface ExportPreviewModalProps {
   onClose: () => void
   content: string
   filename: string
-  format: 'markdown' | 'text' | 'html'
+  format: 'markdown' | 'text' | 'html' | 'psa'
   onDownload: () => void
 }