add session-based connection management and React dashboard

Backend: adds JDBC session support, login/status/logout endpoints, and
new DTOs (AuthResponse, ConnectionStatusResponse, LoginResult). Frontend
replaces the Vite boilerplate with a Dashboard, ServiceCard, and
ConnectModal backed by a typed API client.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -29,6 +29,7 @@
     "eslint-plugin-react-refresh": "^0.5.2",
     "globals": "^17.4.0",
     "jsdom": "^29.0.1",
+    "sass": "^1.99.0",
     "typescript": "~5.9.3",
     "typescript-eslint": "^8.57.0",
     "vite": "^8.0.1",