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/src/standalone.ts

@@ -0,0 +1,8 @@
+// Standalone (non-SvelteKit) entry for the offline keepsake. Mounts the plain
+// Svelte viewer component into a single, self-contained index.html so it works
+// when opened by double-clicking from an extracted ZIP (file://).
+import './app.css';
+import { mount } from 'svelte';
+import App from './routes/+page.svelte';
+
+mount(App, { target: document.getElementById('app')! });