Phase 21: container terminal (web exec), local + agent (0.27.0)

Interactive shell into a compose-managed container over WebSocket + xterm.js,
opened from the container card on the stack Overview tab. Admin-only (non-admin
handshake rejected with 4403); only containers with the compose project label
are reachable.

- backend services/exec_service.py: create/start/resize exec + a shared
  bidirectional pump_exec (recv/sendall on sock._sock, executor thread,
  resize control frames, exit-code frame).
- routers/ws.py: _authorize_admin + /ws/exec/{container_id} and the
  /ws/agent-exec/{agent_id}/{container_id} proxy (forwards BOTH directions).
- agent_app.py: /agent/ws/exec/{container_id}.
- frontend: @xterm/xterm + @xterm/addon-fit; ContainerTerminal modal (shell
  picker, fit/resize, exit/error handling) + a Terminal button on ContainerCard.

Live-verified (TestClient): local happy/exit/guard/4403/4401, agent happy/4401,
proxy bidirectional round-trip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/agent_app.py

@@ -42,6 +42,7 @@ from services import (
     compose_service,
     container_service,
     device_service,
+    exec_service,
     file_service,
     image_service,
     network_service,
@@ -63,7 +64,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.26.0"
+AGENT_VERSION = "0.27.0"
 
 
 # --------------------------------------------------------------------------- #
@@ -693,6 +694,41 @@ async def ws_deploy(websocket: WebSocket, stack_id: str, token: str | None = Que
         compose_service.clear_busy(stack_id)
 
 
+@app.websocket("/agent/ws/exec/{container_id}")
+async def ws_exec(
+    websocket: WebSocket,
+    container_id: str,
+    token: str | None = Query(default=None),
+    cmd: str | None = Query(default=None),
+):
+    """Interactive shell into a compose-managed container (token via query)."""
+    await websocket.accept()
+    expected = settings.AGENT_TOKEN
+    if not expected or token != expected:
+        await websocket.close(code=4401)
+        return
+    shell = cmd or exec_service.DEFAULT_SHELL
+    try:
+        exec_id = exec_service.create_exec(container_id, [shell])
+        holder, raw = exec_service.start_exec(exec_id)
+    except Exception as exc:  # noqa: BLE001
+        try:
+            await websocket.send_text(json.dumps({"type": "error", "detail": str(exc)}))
+        except Exception:  # noqa: BLE001
+            pass
+        await websocket.close()
+        return
+    try:
+        await exec_service.pump_exec(websocket, exec_id, holder, raw)
+    except WebSocketDisconnect:
+        pass
+    finally:
+        try:
+            await websocket.close()
+        except Exception:  # noqa: BLE001
+            pass
+
+
 @app.get("/agent/health")
 def health() -> dict:
     return {"status": "ok"}