feat(escalations): magic-moment 3-option CTA + claim 500 fix

- HandoffContextScreen: 3-option layout (Continue/AI analysis/Own thing)
  with hasTaskLane, activeOptionKey, spinner/disabled states
- AssistantChatPage: wire up handleContinue, handleAIAnalysis, handleOwnThing
  handlers; chip detail expansion inline with copy-button fix; post-escalation
  redirect to dashboard on ConcludeSessionModal close
- TaskLane: fix async copy button (await + execCommand fallback + copiedKey
  visual feedback); whitespace-pre-wrap on command blocks
- Fix 500 on claim: Pydantic v2 model_validate() + model_copy(update={})
  (was passing update= kwarg directly which v2 rejects)
- HandoffResponse schema: handed_off_by_name field

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

backend/app/api/endpoints/session_handoffs.py

@@ -90,7 +90,9 @@ async def create_handoff(
             enrich_escalation_async, handoff.id, current_user.id
         )
 
-    return HandoffResponse.model_validate(handoff)
+    return HandoffResponse.model_validate(handoff).model_copy(
+        update={"handed_off_by_name": current_user.name}
+    )
 
 
 @router.get("/handoffs", response_model=list[HandoffResponse])
@@ -146,7 +148,14 @@ async def claim_handoff(
         raise HTTPException(status_code=404, detail=str(e))
 
     await db.commit()
-    return HandoffResponse.model_validate(handoff)
+    handed_off_by_name = (
+        handoff.handed_off_by_user.name
+        if handoff.handed_off_by_user
+        else None
+    )
+    return HandoffResponse.model_validate(handoff).model_copy(
+        update={"handed_off_by_name": handed_off_by_name}
+    )
 
 
 @queue_router.get("/queue")