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/api/src/main.py

@@ -11,6 +11,7 @@ from src.api.dashboard import router as dashboard_router
 from src.api.git_repositories import router as git_repositories_router
 from src.api.projects import router as projects_router
 from src.api.ssh_keys import router as ssh_keys_router
+from src.api.terminal import router as terminal_router
 from src.api.tool_instances import router as tool_instances_router
 from src.api.tool_instances import sessions_router
 from src.api.tool_types import router as tool_types_router
@@ -168,4 +169,5 @@ app.include_router(user_config_router)
 app.include_router(tool_types_router)
 app.include_router(tool_instances_router)
 app.include_router(sessions_router)
+app.include_router(terminal_router)
 app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")