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.standalone.config.js

@@ -0,0 +1,25 @@
+import { svelte, vitePreprocess } from '@sveltejs/vite-plugin-svelte';
+import tailwindcss from '@tailwindcss/vite';
+import { viteSingleFile } from 'vite-plugin-singlefile';
+import { defineConfig } from 'vite';
+import { fileURLToPath } from 'node:url';
+
+// Builds the keepsake viewer as ONE self-contained index.html (all JS + CSS
+// inlined) so it renders when opened via file://. Uses the plain Svelte plugin
+// (not SvelteKit) because SvelteKit emits multiple module entry points, which
+// cannot be inlined into a single file.
+export default defineConfig({
+	plugins: [
+		tailwindcss(),
+		svelte({ configFile: false, preprocess: vitePreprocess(), compilerOptions: { runes: true } }),
+		viteSingleFile()
+	],
+	resolve: {
+		alias: { $lib: fileURLToPath(new URL('./src/lib', import.meta.url)) }
+	},
+	build: {
+		outDir: fileURLToPath(new URL('../../backend/static/export-viewer', import.meta.url)),
+		emptyOutDir: true,
+		target: 'es2020'
+	}
+});