fix(purchases): persist verbose log toggle, fix dropdown stacking, wire Add to Library button

- Verbose capture toggle now persists to localStorage across sessions
- PurchasesView dropdown menus now have mutual exclusion (opening one closes others)
- Add to Library button wired to v2_library_add_folder with download destination tracking
- Added i18n keys for add-to-library success/error toasts (all 4 locales)

src/lib/i18n/locales/fr.json

@@ -1446,6 +1446,8 @@
     "purchasedOn": "Acheté :",
     "addToLibrary": "Ajouter à la bibliothèque locale",
     "addToLibraryDesc": "Ajouter le dossier téléchargé à votre bibliothèque musicale locale.",
+    "addToLibrarySuccess": "Dossier ajoute a la bibliotheque locale.",
+    "addToLibraryError": "Erreur lors de l'ajout du dossier a la bibliotheque.",
     "downloading": "Téléchargement...",
     "downloadProgress": "Téléchargement {current}/{total}...",
     "complete": "Téléchargement terminé",