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/lib/types.ts

@@ -0,0 +1,28 @@
+export interface ViewerData {
+	event: {
+		name: string;
+		exported_at: string;
+	};
+	posts: ViewerPost[];
+}
+
+export interface ViewerPost {
+	id: string;
+	uploader: string;
+	caption: string;
+	tags: string[];
+	timestamp: string;
+	likes: number;
+	comments: ViewerComment[];
+	media: {
+		type: 'image' | 'video';
+		thumb: string;
+		full: string;
+	};
+}
+
+export interface ViewerComment {
+	author: string;
+	text: string;
+	timestamp: string;
+}