fix(flowpilot): widen message bar, add close/abandon session support

- Message bar now fixed-positioned above action bar with full-width
  layout (respects both app sidebar and session sidebar)
- Added abandon_session endpoint (POST /ai-sessions/{id}/abandon)
- Added "Close" button to FlowPilot action bar with confirmation dialog
- Session can now be closed without resolving or escalating

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/aiSessions.ts

@@ -96,6 +96,12 @@ export const aiSessionsApi = {
     await apiClient.post(`/ai-sessions/${sessionId}/resume`)
   },
 
+  async abandonSession(sessionId: string, reason?: string): Promise<void> {
+    await apiClient.post(`/ai-sessions/${sessionId}/abandon`, null, {
+      params: reason ? { reason } : undefined,
+    })
+  },
+
   async pickupSession(sessionId: string, data: PickupSessionRequest): Promise<StepResponseResponse> {
     const response = await apiClient.post<StepResponseResponse>(
       `/ai-sessions/${sessionId}/pickup`,