feat: implement web terminal for tool instances

- Add TerminalSession backend service for docker exec subprocess management
- Add TerminalManager for WebSocket session lifecycle management
- Create WebSocket endpoint at /ws/tool-instances/{id}/terminal
- Add session cookie authentication and instance ownership verification
- Install xterm.js with fit and web-links addons
- Create TerminalComponent with xterm.js integration
- Create TerminalPage with full-screen terminal view
- Add terminal route at /instances/:id/terminal
- Add terminal button to InstanceList for running instances
- Add terminal and arrow-left icons to icon registry
- Add comprehensive terminal CSS styles (dark theme, responsive)

Quality gates: typecheck ✓, lint ✓, build ✓, Python syntax ✓

apps/web/package.json

@@ -19,7 +19,10 @@
     "react-dom": "^18.2.0",
     "react-router-dom": "^6.20.0",
     "react-simple-code-editor": "^0.14.1",
-    "tailwindcss": "^3.3.0"
+    "tailwindcss": "^3.3.0",
+    "xterm": "^5.3.0",
+    "xterm-addon-fit": "^0.8.0",
+    "xterm-addon-web-links": "^0.9.0"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.9.1",