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/hooks/useFlowPilotSession.ts

@@ -29,6 +29,7 @@ export interface UseFlowPilotSession {
   escalateSession: (data: EscalateSessionRequest) => Promise<SessionDocumentation>
   pauseSession: () => Promise<void>
   resumeOwnSession: () => Promise<void>
+  abandonSession: () => Promise<void>
   rateSession: (rating: number, feedback?: string) => Promise<void>
   loadSession: (sessionId: string) => Promise<void>
 
@@ -190,6 +191,18 @@ export function useFlowPilotSession(): UseFlowPilotSession {
     }
   }, [session])
 
+  const abandonSession = useCallback(async () => {
+    if (!session) return
+    try {
+      await aiSessionsApi.abandonSession(session.id)
+      setSession(prev => prev ? { ...prev, status: 'abandoned' } : null)
+      setCurrentStep(null)
+      toast.success('Session closed')
+    } catch {
+      toast.error('Failed to close session')
+    }
+  }, [session])
+
   const rateSession = useCallback(async (rating: number, feedback?: string) => {
     if (!session) return
     try {
@@ -245,6 +258,7 @@ export function useFlowPilotSession(): UseFlowPilotSession {
     escalateSession,
     pauseSession,
     resumeOwnSession,
+    abandonSession,
     rateSession,
     loadSession,
     isActive,