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>

frontend/src/types/branching.ts

@@ -86,14 +86,17 @@ export interface HandoffResponse {
   id: string
   session_id: string
   handed_off_by: string
+  handed_off_by_name: string | null
   intent: 'park' | 'escalate'
   source_branch_id: string | null
   snapshot: Record<string, unknown>
   ai_assessment: string | null
   ai_assessment_data: {
+    summary_prose?: string
+    what_we_know?: string[]
     likely_cause: string
     suggested_steps: string[]
-    confidence: number
+    confidence: string
   } | null
   artifacts: Array<{
     name: string