feat: PDF-only export with premium download animation

- Replaced dropdown export menu with single PDF download button
- Added full-screen overlay animation during download (shimmer, spinner, success check)
- Removed PWA support (manifest.json, sw.js, service worker registration)
- Removed offline HTML export (vite-plugin-singlefile, vite.export.config.ts, copy-offline.mjs)
- Simplified build script to just 'vite build'
- Regenerated PDF at 3x device scale factor (25.48 MB, 18 slides at 5760x3240 pixels)

Author: CodeCraftsmaniac

.gitignore

@@ -24,3 +24,4 @@ dist-ssr
 *.sw?
 .vercel
 TASKS_V3_Universal_Deck.md
+.pdf-tmp