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/api/connections.ts

@@ -0,0 +1,11 @@
+import { apiFetch } from './client'
+import type { AuthResponse, ConnectionStatus, LoginRequest } from '../types/connection'
+
+export const login = (req: LoginRequest) =>
+  apiFetch<AuthResponse>('/login', { method: 'POST', body: JSON.stringify(req) })
+
+export const getStatuses = () =>
+  apiFetch<ConnectionStatus[]>('/connections/status')
+
+export const logout = (serviceType: string) =>
+  apiFetch<void>(`/connections/${serviceType}`, { method: 'DELETE' })