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/package.json

@@ -0,0 +1,22 @@
+{
+	"name": "export-viewer",
+	"private": true,
+	"version": "0.0.1",
+	"type": "module",
+	"scripts": {
+		"dev": "vite dev",
+		"build": "vite build",
+		"preview": "vite preview",
+		"prepare": "svelte-kit sync || echo ''"
+	},
+	"devDependencies": {
+		"@sveltejs/adapter-static": "^3.0.0",
+		"@sveltejs/kit": "^2.50.2",
+		"@sveltejs/vite-plugin-svelte": "^6.2.4",
+		"@tailwindcss/vite": "^4.2.2",
+		"svelte": "^5.54.0",
+		"tailwindcss": "^4.2.2",
+		"typescript": "^5.9.3",
+		"vite": "^7.3.1"
+	}
+}