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/dto/ConnectionStatusResponse.java

@@ -0,0 +1,10 @@
+package com.vaessl.app.dto;
+
+import java.time.Instant;
+
+public record ConnectionStatusResponse(
+        String serviceType,
+        String appUrl,
+        String username,
+        Instant expiresAt,
+        boolean connected) {}