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/test-images/page.tsx

@@ -48,7 +48,7 @@ export default function TestImagesPage() {
   ];
 
   return (
-    <main className="container mx-auto px-4 py-8">
+    <main id="main-content" className="container mx-auto px-4 py-8">
       <div className="mb-8">
         <h1 className="text-4xl font-bold text-secondary dark:text-gray-50">
           Image Source Test Page