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/META-INF/additional-spring-configuration-metadata.json

@@ -0,0 +1,12 @@
+{"properties": [
+  {
+    "name": "spring.session.store-type",
+    "type": "java.lang.String",
+    "description": "A description for 'spring.session.store-type'"
+  },
+  {
+    "name": "vaessl.frontend-url",
+    "type": "java.lang.String",
+    "description": "A description for 'vaessl.frontend-url'"
+  }
+]}