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>

backend/app/schemas/session.py

@@ -70,7 +70,7 @@ class SessionResponse(BaseModel):
 
 
 class SessionExport(BaseModel):
-    format: str = Field(default="markdown", pattern="^(text|markdown|html)$")
+    format: str = Field(default="markdown", pattern="^(text|markdown|html|psa)$")
     include_timestamps: bool = True
     include_tree_info: bool = True