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/tsconfig.json

@@ -0,0 +1,15 @@
+{
+	"extends": "./.svelte-kit/tsconfig.json",
+	"compilerOptions": {
+		"rewriteRelativeImportExtensions": true,
+		"allowJs": true,
+		"checkJs": true,
+		"esModuleInterop": true,
+		"forceConsistentCasingInFileNames": true,
+		"resolveJsonModule": true,
+		"skipLibCheck": true,
+		"sourceMap": true,
+		"strict": true,
+		"moduleResolution": "bundler"
+	}
+}