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/java/com/vaessl/app/connection/Endpoint.java

@@ -1,7 +1,9 @@
 package com.vaessl.app.connection;
 
 public enum Endpoint {
-    HOMEBOX_LOGIN("/api/v1/users/login"), LOGIN("/login");
+    HOMEBOX_LOGIN("/api/v1/users/login"),
+    LOGIN("/login"),
+    CONNECTION_STATUS("/connections/status");
 
     private final String value;