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/package.json

@@ -5,7 +5,8 @@
 	"type": "module",
 	"scripts": {
 		"dev": "vite dev",
-		"build": "vite build",
+		"build": "vite build --config vite.standalone.config.js",
+		"build:sveltekit": "vite build",
 		"preview": "vite preview",
 		"prepare": "svelte-kit sync || echo ''"
 	},
@@ -17,6 +18,7 @@
 		"svelte": "^5.54.0",
 		"tailwindcss": "^4.2.2",
 		"typescript": "^5.9.3",
-		"vite": "^7.3.1"
+		"vite": "^7.3.1",
+		"vite-plugin-singlefile": "^2.3.3"
 	}
 }