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>

backend/app/api/endpoints/ai_sessions.py

@@ -396,6 +396,34 @@ async def resume_session(
     await db.commit()
 
 
+# ── Abandon / Close ──
+
+@router.post("/{session_id}/abandon", status_code=204)
+@limiter.limit("15/minute")
+async def abandon_session(
+    request: Request,
+    session_id: UUID,
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+    _: None = Depends(require_engineer_or_admin),
+    reason: str | None = None,
+):
+    """Close a session without resolving or escalating."""
+    try:
+        await flowpilot_engine.abandon_session(
+            session_id=session_id,
+            user_id=current_user.id,
+            reason=reason,
+            db=db,
+        )
+    except ValueError as e:
+        raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail=str(e))
+    except PermissionError as e:
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=str(e))
+
+    await db.commit()
+
+
 # ── Escalation Queue ──
 
 @router.get("/escalation-queue", response_model=list[AISessionSummary])