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/components/flowpilot/FlowPilotSession.tsx

@@ -36,6 +36,7 @@ interface FlowPilotSessionProps {
   onEscalate: (data: EscalateSessionRequest) => Promise<SessionDocumentation>
   onPause?: () => Promise<void>
   onResume?: () => Promise<void>
+  onAbandon?: () => Promise<void>
   onRate: (rating: number) => void
   onReloadSession?: () => Promise<void>
 }
@@ -56,6 +57,7 @@ export function FlowPilotSession({
   onEscalate,
   onPause,
   onResume,
+  onAbandon,
   onRate,
   onReloadSession,
 }: FlowPilotSessionProps) {
@@ -321,6 +323,7 @@ export function FlowPilotSession({
           onResolve={onResolve}
           onEscalate={onEscalate}
           onPause={onPause}
+          onAbandon={onAbandon}
         />
       )}