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>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.26.0",
+  "version": "0.27.0",
   "type": "module",
   "scripts": {
     "dev": "vite",
@@ -11,6 +11,8 @@
   "dependencies": {
     "@monaco-editor/react": "^4.6.0",
     "@tanstack/react-query": "^5.62.7",
+    "@xterm/addon-fit": "^0.10.0",
+    "@xterm/xterm": "^5.5.0",
     "axios": "^1.7.9",
     "clsx": "^2.1.1",
     "lucide-react": "^0.468.0",