feat(playlists): add ZIP download button to playlist detail page

Mirrors the album download UX: Download (ZIP) button in the playlist
hero header, progress bar while downloading, folder picker via
downloadModalStore (remembers last used folder). Uses the same
Subsonic /rest/download.view endpoint as album downloads — it accepts
playlist IDs just as well as album IDs.

Closes #127

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/fr.ts

@@ -775,6 +775,7 @@ export const frTranslation = {
     removeCover: 'Supprimer la photo',
     coverUpdated: 'Pochette mise à jour',
     metaSaved: 'Playlist mise à jour',
+    downloadZip: 'Télécharger (ZIP)',
   },
   radio: {
     title: 'Radio Internet',