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/vite.config.ts

@@ -0,0 +1,7 @@
+import { sveltekit } from '@sveltejs/kit/vite';
+import tailwindcss from '@tailwindcss/vite';
+import { defineConfig } from 'vite';
+
+export default defineConfig({
+	plugins: [tailwindcss(), sveltekit()]
+});