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/tests/test_session_handoffs_api.py

@@ -23,16 +23,15 @@ def stub_ai_assessment():
     """Endpoint tests should not wait on the external AI assessment path."""
     with patch.object(
         HandoffManager,
-        "_generate_ai_assessment",
+        "_generate_handoff_summary",
         new=AsyncMock(
-            return_value=(
-                "Stub escalation assessment",
-                {
-                    "likely_cause": "Stub",
-                    "suggested_steps": [],
-                    "confidence": "medium",
-                },
-            )
+            return_value={
+                "summary_prose": "Stub escalation assessment",
+                "what_we_know": [],
+                "likely_cause": "Stub",
+                "suggested_steps": [],
+                "confidence": "medium",
+            }
         ),
     ):
         yield