New notebook logo: spiral-bound page icon for favicon/PWA/desktop

SVG source in frontend/public/icon.svg, rendered to 192/512/maskable PNGs
and desktop/build/appicon.png; SVG favicon with PNG fallback; icons added
to the service-worker precache.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/vite.config.ts

@@ -14,7 +14,7 @@ function serviceWorker(): Plugin {
       const assets = Object.keys(bundle)
         .filter((f) => !f.endsWith('.map'))
         .map((f) => '/' + f);
-      assets.push('/', '/manifest.webmanifest', '/icon-192.png', '/icon-512.png');
+      assets.push('/', '/manifest.webmanifest', '/icon.svg', '/icon-192.png', '/icon-512.png', '/icon-512-maskable.png');
       const hash = createHash('sha256').update(assets.join(',') + version).digest('hex').slice(0, 12);
       const sw = `// generated by vite.config.ts — do not edit
 const CACHE = 'tefter-${'${'}HASH}';