feat: add quality/format selector for individual track downloads

When downloading a single track from the Purchases tab, if multiple
formats are available a popup lets the user choose before the folder
picker opens. Single-format tracks proceed directly as before.

src/lib/i18n/locales/fr.json

@@ -1554,6 +1554,7 @@
     "downloadAll": "Tout télécharger",
     "downloadTrack": "Télécharger la piste",
     "chooseFolder": "Choisir le dossier de téléchargement",
+    "selectFormat": "Sélectionner le format",
     "format": "Format",
     "purchasedOn": "Acheté :",
     "addToLibrary": "Ajouter à la bibliothèque locale",