feat(client): add accessible design system and API client foundation

Add design tokens, base components (page header, section, status chip, empty state, loading, announcer), a11y composables (text scaling, announcements), high-contrast theme and a typed HTTP client with ProblemDetails error mapping.

client/src/services/api/client.ts

@@ -0,0 +1,72 @@
+import { ApiError, type ProblemDetails } from './errors'
+
+const baseUrl = import.meta.env.VITE_API_BASE_URL ?? ''
+
+let tokenProvider: () => string | null = () => null
+
+/** Wires the API client to the session store without creating a hard dependency. */
+export function setAuthTokenProvider(provider: () => string | null): void {
+  tokenProvider = provider
+}
+
+export interface RequestOptions {
+  method?: 'GET' | 'POST' | 'PUT' | 'PATCH' | 'DELETE'
+  body?: unknown
+  headers?: Record<string, string>
+  signal?: AbortSignal
+  /** Attach the bearer token when available (default: true). */
+  auth?: boolean
+}
+
+async function parseBody(response: Response): Promise<unknown> {
+  if (response.status === 204 || response.headers.get('content-length') === '0') {
+    return undefined
+  }
+
+  const contentType = response.headers.get('content-type') ?? ''
+
+  if (!contentType.includes('application/json')) {
+    return undefined
+  }
+
+  try {
+    return await response.json()
+  } catch {
+    return undefined
+  }
+}
+
+/**
+ * Minimal typed fetch wrapper: JSON in/out, bearer auth, RFC 7807 error mapping.
+ * Throws `ApiError` on non-2xx responses.
+ */
+export async function request<T>(path: string, options: RequestOptions = {}): Promise<T> {
+  const { method = 'GET', body, headers = {}, signal, auth = true } = options
+
+  const requestHeaders: Record<string, string> = { Accept: 'application/json', ...headers }
+  const token = auth ? tokenProvider() : null
+
+  if (token) {
+    requestHeaders.Authorization = `Bearer ${token}`
+  }
+
+  if (body !== undefined) {
+    requestHeaders['Content-Type'] = 'application/json'
+  }
+
+  const response = await fetch(`${baseUrl}${path}`, {
+    method,
+    headers: requestHeaders,
+    body: body === undefined ? undefined : JSON.stringify(body),
+    signal,
+  })
+
+  const payload = await parseBody(response)
+
+  if (!response.ok) {
+    const problem = (payload as ProblemDetails | undefined) ?? null
+    throw new ApiError(response.status, problem)
+  }
+
+  return payload as T
+}