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/src/router.tsx

@@ -12,6 +12,7 @@ import { ProjectSettingsPage } from "./pages/project-settings";
 import { RepoWorkspace } from "./pages/repo-workspace";
 import { SSHKeysPage } from "./pages/ssh-keys";
 import { SettingsPage } from "./pages/settings";
+import { TerminalPage } from "./pages/terminal";
 import { ToolTypesPage } from "./pages/tool-types";
 
 export const AppRouter = () => {
@@ -36,6 +37,7 @@ export const AppRouter = () => {
         <Route path="profile" element={<ProfilePage />} />
         <Route path="settings" element={<SettingsPage />} />
         <Route path="tool-types" element={<ToolTypesPage />} />
+        <Route path="instances/:instanceId/terminal" element={<TerminalPage />} />
       </Route>
       <Route path="/404" element={<NotFoundPage />} />
       <Route path="*" element={<Navigate to="/404" replace />} />