feat(export): single-file offline keepsake viewer + guest download

Rebuild the guest "keepsake" (offline HTML gallery) so it renders when the
extracted index.html is opened over file://. Browsers block external ES-module
scripts and fetch() at origin null, so a normal multi-file SvelteKit build shows
a blank window. Build the viewer as one self-contained index.html (inlined
JS/CSS via vite-plugin-singlefile, standalone non-SvelteKit entry) and inject
the data as window.__EXPORT_DATA__; the backend embeds the built viewer via
include_dir! and writes the data global into index.html when zipping.

Also surface the guest-facing download: a feed banner and the BottomNav Export
tab, both shown once the host releases the export.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/export-viewer/vite.config.js

@@ -0,0 +1,10 @@
+import { sveltekit } from '@sveltejs/kit/vite';
+import tailwindcss from '@tailwindcss/vite';
+import { defineConfig } from 'vite';
+
+// NOTE: the production keepsake is built with `vite.standalone.config.js`
+// (see `npm run build`), which inlines everything into a single, file://-safe
+// index.html. This SvelteKit config is kept only for `npm run dev`/`preview`.
+export default defineConfig({
+	plugins: [tailwindcss(), sveltekit()]
+});