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/services/flowpilot_engine.py

@@ -793,6 +793,29 @@ async def resume_session(
     await db.flush()
 
 
+async def abandon_session(
+    session_id: UUID,
+    user_id: UUID,
+    reason: Optional[str],
+    db: AsyncSession,
+) -> None:
+    """Close a session without resolving or escalating.
+
+    Used when the engineer no longer needs help, figured it out on their own,
+    or the session is no longer relevant.
+    """
+    session = await _load_session(session_id, user_id, db)
+
+    if session.status not in ("active", "paused"):
+        raise ValueError(f"Cannot close session in status: {session.status}")
+
+    session.status = "abandoned"
+    session.resolved_at = datetime.now(timezone.utc)
+    if reason:
+        session.resolution_notes = reason
+    await db.flush()
+
+
 async def rate_session(
     session_id: UUID,
     rating: int,