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/packages/a2ui-runtime/core/A2UITypes.ts

@@ -116,6 +116,7 @@ export const CheckboxComponentSchema = z.object({
     checked: BooleanContentSchema.optional(),
     onChange: ActionSchema,
     label: TextContentSchema.optional(),
+    description: TextContentSchema.optional(),
   }),
 });
 
@@ -202,6 +203,8 @@ export const ComponentSchema: z.ZodType<any> = z.union([
 export const SurfaceComponentSchema = z.object({
   id: z.string(),
   component: ComponentSchema,
+  /** Page index for multi-page surfaces (0-based) */
+  page: z.number().int().min(0).optional(),
 });
 
 /** Display mode for A2UI surfaces */

ccw/frontend/src/packages/a2ui-runtime/renderer/components/A2UICheckbox.tsx

@@ -51,17 +51,28 @@ export const A2UICheckbox: ComponentRenderer = ({ component, state, onAction, re
     ? resolveTextContent(checkboxConfig.label, resolveBinding)
     : '';
 
+  // Resolve description text
+  const descriptionText = checkboxConfig.description
+    ? resolveTextContent(checkboxConfig.description, resolveBinding)
+    : '';
+
   return (
-    <div className="flex items-center space-x-2">
+    <div className="flex items-start space-x-2">
       <Checkbox
+        className="mt-0.5"
         checked={checked}
         onCheckedChange={handleChange}
       />
-      {labelText && (
-        <Label className="text-sm font-medium leading-none peer-disabled:cursor-not-allowed peer-disabled:opacity-70">
-          {labelText}
-        </Label>
-      )}
+      <div className="grid gap-0.5">
+        {labelText && (
+          <Label className="text-sm font-medium leading-none peer-disabled:cursor-not-allowed peer-disabled:opacity-70">
+            {labelText}
+          </Label>
+        )}
+        {descriptionText && (
+          <p className="text-xs text-muted-foreground">{descriptionText}</p>
+        )}
+      </div>
     </div>
   );
 };