feat: add dark mode and audit viewer-frontend accessibility

Add next-themes light/dark/system theming with an accessible toggle
in every header, and close out an a11y pass across the viewer app:
skip-to-content link, main landmarks and heading hierarchy on every
route, focus-trapped dialog semantics for the photo viewer modals,
aria-label/live-region coverage for dynamic messages and icon
buttons, and dark: variants for previously light-only colors.

viewer-frontend/app/HomePageContent.tsx

@@ -997,8 +997,8 @@ export function HomePageContent({ initialPhotos, people, tags }: HomePageContent
         />
         <div className="flex-1 min-w-0">
           {loading ? (
-            <div className="flex items-center justify-center py-12">
-              <Loader2 className="h-8 w-8 animate-spin text-gray-400" />
+            <div role="status" aria-label="Loading photos" className="flex items-center justify-center py-12">
+              <Loader2 className="h-8 w-8 animate-spin text-gray-400 dark:text-gray-500" />
             </div>
           ) : (
             <>
@@ -1046,9 +1046,9 @@ export function HomePageContent({ initialPhotos, people, tags }: HomePageContent
               />
               
               {/* Infinite scroll sentinel */}
-              <div ref={observerTarget} className="h-10 flex items-center justify-center">
+              <div ref={observerTarget} role="status" aria-label={loadingMore ? 'Loading more photos' : undefined} className="h-10 flex items-center justify-center">
                 {loadingMore && (
-                  <Loader2 className="h-6 w-6 animate-spin text-gray-400" />
+                  <Loader2 className="h-6 w-6 animate-spin text-gray-400 dark:text-gray-500" />
                 )}
               </div>