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/page.tsx

@@ -222,10 +222,10 @@ export default async function HomePage() {
   ]);
 
   return (
-    <main className="w-full px-4 py-8">
+    <main id="main-content" className="w-full px-4 pt-3 pb-8">
 
       {error ? (
-        <div className="rounded-lg bg-red-50 p-4 text-red-800 dark:bg-red-900/20 dark:text-red-200">
+        <div role="alert" className="rounded-lg bg-red-50 p-4 text-red-800 dark:bg-red-900/20 dark:text-red-200">
           <p className="font-semibold">Error loading photos</p>
           <p className="text-sm">{error}</p>
           <p className="mt-2 text-xs">