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/ConnectionService.java

@@ -1,5 +1,6 @@
 package com.vaessl.app.connection;
 
+import java.time.Instant;
 import java.util.List;
 import java.util.Map;
 import java.util.stream.Collectors;
@@ -8,6 +9,8 @@ import org.springframework.stereotype.Service;
 
 import com.vaessl.app.dto.ConnectionRequest;
 import com.vaessl.app.dto.ConnectionResponse;
+import com.vaessl.app.dto.ConnectionStatusResponse;
+import com.vaessl.app.dto.LoginResult;
 import com.vaessl.app.exception.WrongServiceTypeException;
 
 @Service
@@ -23,7 +26,7 @@ public class ConnectionService {
         this.cRepository = cRepository;
     }
 
-    public ConnectionResponse login(ConnectionRequest request) {
+    public LoginResult login(ConnectionRequest request) {
 
         ConnectionProvider provider = providerRegistry.get(request.serviceType());
 
@@ -37,13 +40,27 @@ public class ConnectionService {
 
         ConnectionEntity existing = provider.findUniqueConnectionEntry(request);
 
+        ConnectionEntity saved;
         if (existing != null) {
             provider.updateToRepository(existing, response);
+            saved = existing;
         } else {
             ConnectionEntity newEntity = provider.connectionToEntity(request, response);
-            cRepository.save(newEntity);
+            saved = cRepository.save(newEntity);
         }
 
-        return response;
+        return new LoginResult(saved.getId(), response.expiresAt());
     }
-}
+
+    public ConnectionStatusResponse getConnectionStatus(String serviceType, Long connectionId) {
+        ConnectionEntity entity = cRepository.findById(connectionId).orElse(null);
+        if (entity == null) return null;
+
+        ConnectionProvider provider = providerRegistry.get(serviceType);
+        Instant expiresAt = (provider != null) ? provider.getTokenExpiry(entity) : null;
+        boolean connected = expiresAt == null || expiresAt.isAfter(Instant.now());
+
+        return new ConnectionStatusResponse(serviceType, entity.getAppUrl(),
+                entity.getUsername(), expiresAt, connected);
+    }
+}