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/client.ts

@@ -0,0 +1,21 @@
+const BASE = import.meta.env.VITE_API_URL ?? '/api'
+
+export async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
+  const res = await fetch(`${BASE}${path}`, {
+    ...init,
+    credentials: 'include',
+    headers: { 'Content-Type': 'application/json', ...init?.headers },
+  })
+  if (!res.ok) {
+    let message = `Request failed (${res.status})`
+    try {
+      const body = await res.json()
+      if (body?.detail) message = body.detail
+      else if (body?.title) message = body.title
+    } catch {
+      // ignore parse errors
+    }
+    throw new Error(message)
+  }
+  return res.status === 204 ? (undefined as T) : res.json()
+}