feat: add Unsplash search hook and API proxy routes

- Implemented `useUnsplashSearch` hook for searching Unsplash photos with debounce.
- Created Unsplash API client functions for searching photos and triggering downloads.
- Added proxy routes for Unsplash API to handle search requests and background image uploads.
- Introduced accessibility utilities for WCAG compliance checks and motion preference management.
- Developed theme sharing module for encoding and decoding theme configurations as base64url strings.

ccw/frontend/src/lib/unsplash.ts

@@ -0,0 +1,102 @@
+/**
+ * Unsplash API Client
+ * Frontend functions to search Unsplash via the backend proxy.
+ */
+
+export interface UnsplashPhoto {
+  id: string;
+  thumbUrl: string;
+  smallUrl: string;
+  regularUrl: string;
+  photographer: string;
+  photographerUrl: string;
+  photoUrl: string;
+  blurHash: string | null;
+  downloadLocation: string;
+}
+
+export interface UnsplashSearchResult {
+  photos: UnsplashPhoto[];
+  total: number;
+  totalPages: number;
+}
+
+function getCsrfToken(): string | null {
+  const match = document.cookie.match(/XSRF-TOKEN=([^;]+)/);
+  return match ? decodeURIComponent(match[1]) : null;
+}
+
+/**
+ * Search Unsplash photos via backend proxy.
+ */
+export async function searchUnsplash(
+  query: string,
+  page = 1,
+  perPage = 20
+): Promise<UnsplashSearchResult> {
+  const params = new URLSearchParams({
+    query,
+    page: String(page),
+    per_page: String(perPage),
+  });
+
+  const response = await fetch(`/api/unsplash/search?${params}`, {
+    credentials: 'same-origin',
+  });
+
+  if (!response.ok) {
+    const body = await response.json().catch(() => ({}));
+    throw new Error(body.error || `Unsplash search failed: ${response.status}`);
+  }
+
+  return response.json();
+}
+
+/**
+ * Upload a local image as background.
+ * Sends raw binary to avoid base64 overhead.
+ */
+export async function uploadBackgroundImage(file: File): Promise<{ url: string; filename: string }> {
+  const headers: Record<string, string> = {
+    'Content-Type': file.type,
+    'X-Filename': file.name,
+  };
+  const csrfToken = getCsrfToken();
+  if (csrfToken) {
+    headers['X-CSRF-Token'] = csrfToken;
+  }
+
+  const response = await fetch('/api/background/upload', {
+    method: 'POST',
+    headers,
+    credentials: 'same-origin',
+    body: file,
+  });
+
+  if (!response.ok) {
+    const body = await response.json().catch(() => ({}));
+    throw new Error(body.error || `Upload failed: ${response.status}`);
+  }
+
+  return response.json();
+}
+
+/**
+ * Trigger Unsplash download event (required by API guidelines).
+ */
+export async function triggerUnsplashDownload(downloadLocation: string): Promise<void> {
+  const headers: Record<string, string> = {
+    'Content-Type': 'application/json',
+  };
+  const csrfToken = getCsrfToken();
+  if (csrfToken) {
+    headers['X-CSRF-Token'] = csrfToken;
+  }
+
+  await fetch('/api/unsplash/download', {
+    method: 'POST',
+    headers,
+    credentials: 'same-origin',
+    body: JSON.stringify({ downloadLocation }),
+  });
+}