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/pages/FlowPilotSessionPage.tsx

@@ -230,6 +230,7 @@ export default function FlowPilotSessionPage() {
           onEscalate={fp.escalateSession}
           onPause={fp.pauseSession}
           onResume={fp.resumeOwnSession}
+          onAbandon={fp.abandonSession}
           onRate={fp.rateSession}
           onReloadSession={() => fp.loadSession(fp.session!.id)}
         />