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>

backend/src/main/resources/application.yaml

@@ -22,6 +22,12 @@ spring:
       chat:
         options:
           model: gpt-4o-mini
+  session:
+    store-type: jdbc
+    jdbc:
+      initialize-schema: always
 server:
   servlet:
     context-path: /api
+vaessl:
+  frontend-url: ${FRONTEND_URL:http://192.168.1.208:5173/}