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/config/SessionConfig.java

@@ -0,0 +1,19 @@
+package com.vaessl.app.config;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.session.web.http.CookieSerializer;
+import org.springframework.session.web.http.DefaultCookieSerializer;
+
+@Configuration
+public class SessionConfig {
+
+    @Bean
+    public CookieSerializer cookieSerializer() {
+        DefaultCookieSerializer serializer = new DefaultCookieSerializer();
+        serializer.setCookieMaxAge(Integer.MAX_VALUE);
+        serializer.setUseHttpOnlyCookie(true);
+        serializer.setSameSite("Lax");
+        return serializer;
+    }
+}