fix(terminal): prevent xterm.js crash, websocket disconnect cascade, stale sessions

Three related bugs fixed:

1. Frontend xterm.js crash: TerminalPage rendered ALL sessions with display:none
   for inactive ones. xterm.js crashes when initialized in a hidden container
   (Viewport can't read dimensions). Fix: only render the active session's
   TerminalComponent using conditional rendering.

2. Backend websocket disconnect cascade: When client disconnected (due to #1),
   the server tried to send 'connected' status on dead socket, caught the
   WebSocketDisconnect in a generic except block, then tried to close() again
   causing RuntimeError. Fix: catch WebSocketDisconnect specifically and suppress
   close() errors.

3. Stale DB sessions: After server restart, DB still had old terminal session
   rows but no in-memory sessions. list_terminal_sessions returned these ghosts,
   causing the frontend to render dead tabs. Fix: skip DB-only sessions that
   have no live in-memory counterpart.

Quality gates: pytest (15/15 passed), tsc clean, vitest (7/7 passed)

apps/api/src/api/terminal.py

@@ -9,6 +9,7 @@ from contextlib import suppress
 from fastapi import APIRouter, Depends, HTTPException, WebSocket, status
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
+from starlette.websockets import WebSocketDisconnect
 
 from src.auth.dependencies import get_current_user_id, get_db_session
 from src.models.terminal_session import TerminalSessionModel
@@ -207,6 +208,8 @@ async def _handle_terminal_websocket(
         for task in pending:
             task.cancel()
 
+    except WebSocketDisconnect:
+        logger.debug("WebSocket disconnected for instance %s", instance_id)
     except Exception as exc:
         logger.error(
             "Terminal session error for instance %s: %s",
@@ -214,7 +217,8 @@ async def _handle_terminal_websocket(
             str(exc),
             exc_info=True,
         )
-        await websocket.close(code=4000, reason=f"Error: {exc}")
+        with suppress(Exception):
+            await websocket.close(code=4000, reason=f"Error: {exc}")
     finally:
         # Detach WebSocket, don't kill session
         with suppress(Exception):
@@ -403,18 +407,20 @@ async def list_terminal_sessions(
     )
     db_rows = result.scalars().all()
 
-    # Build response with live has_websockets flag
+    # Build response with live has_websockets flag.
+    # Skip DB-only sessions that have no live in-memory counterpart
+    # (e.g. after server restart).
     sessions = []
     for row in db_rows:
         live_session = terminal_manager.get_session(str(instance_id), str(row.id))
+        if not live_session:
+            continue
         sessions.append(
             {
                 "id": str(row.id),
                 "name": row.name,
                 "status": row.status,
-                "has_websockets": live_session.has_websockets()
-                if live_session
-                else False,
+                "has_websockets": live_session.has_websockets(),
                 "created_at": row.created_at.isoformat() if row.created_at else None,
                 "last_activity_at": row.last_activity_at.isoformat()
                 if row.last_activity_at