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/svelte.config.js

@@ -0,0 +1,21 @@
+import adapter from '@sveltejs/adapter-static';
+
+/** @type {import('@sveltejs/kit').Config} */
+const config = {
+	compilerOptions: {
+		runes: true
+	},
+	kit: {
+		adapter: adapter({
+			pages: '../../backend/static/export-viewer',
+			assets: '../../backend/static/export-viewer',
+			fallback: 'index.html',
+			strict: false
+		}),
+		paths: {
+			relative: true
+		}
+	}
+};
+
+export default config;