feat: add export-viewer SvelteKit static app

Standalone SvelteKit project at frontend/export-viewer/ using
adapter-static. Replicates the live feed experience as a read-only
offline gallery: list/grid views, search with autocomplete, hashtag
filtering, lightbox with swipe navigation and comments.

Built output goes to backend/static/export-viewer/ for embedding
into the HTML export ZIP.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/export-viewer/src/routes/+layout.svelte

@@ -0,0 +1,6 @@
+<script>
+	import '../app.css';
+	let { children } = $props();
+</script>
+
+{@render children()}