feat: add booklet download button, remove pdfjs-dist dependency

- Add save/download button to booklet toolbar with native file dialog
- Add v2_booklet_save command to copy PDF to user-chosen location
- Remove pdfjs-dist npm package (replaced by MuPDF backend)
- Delete static/pdfjs/ files (~1.9MB: worker, WASM decoders, fallbacks)
- Add bookletDownload i18n keys to all 4 locales

package.json

@@ -30,7 +30,6 @@
     "@tauri-apps/plugin-dialog": "^2.5.0",
     "@tauri-apps/plugin-opener": "^2",
     "lucide-svelte": "^0.562.0",
-    "pdfjs-dist": "^5.4.624",
     "svelte-i18n": "^4.0.0"
   },
   "devDependencies": {