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>

frontend/src/types/connection.ts

@@ -0,0 +1,20 @@
+export interface LoginRequest {
+  appUrl: string
+  serviceType: string
+  username: string
+  password: string
+  stayLoggedIn: boolean
+}
+
+export interface AuthResponse {
+  serviceType: string
+  expiresAt: string
+}
+
+export interface ConnectionStatus {
+  serviceType: string
+  appUrl: string
+  username: string
+  expiresAt: string | null
+  connected: boolean
+}