feat: /subscribe pricing page + Pro nav link + fix checkout token scope

- Add /subscribe route with hero, benefits list, and pricing cards
  (annual featured with Save 33% badge, monthly secondary)
- Add 'Pro' link in nav for non-Pro users
- Add 'See plans' link in profile subscription section
- i18n keys across en/fr/id/pt/ru for all subscribe strings

Note: checkout still requires POLAR_API_TOKEN with checkouts:write scope.
Regenerate the token at polar.sh to fix the 502 error on subscribe buttons.

ui/messages/fr.json

@@ -366,6 +366,26 @@
   "profile_upgrade_monthly": "Mensuel — 6 $ / mois",
   "profile_upgrade_annual": "Annuel — 48 $ / an",
   "profile_free_limits": "Plan gratuit : 3 chapitres audio par jour, lecture en anglais uniquement.",
+  "subscribe_page_title": "Passer Pro \u2014 libnovel",
+  "subscribe_heading": "Lisez plus. Écoutez plus.",
+  "subscribe_subheading": "Passez Pro et débloquez l'expérience libnovel complète.",
+  "subscribe_monthly_label": "Mensuel",
+  "subscribe_monthly_price": "6 $",
+  "subscribe_monthly_period": "par mois",
+  "subscribe_annual_label": "Annuel",
+  "subscribe_annual_price": "48 $",
+  "subscribe_annual_period": "par an",
+  "subscribe_annual_save": "Économisez 33 %",
+  "subscribe_cta_monthly": "Commencer le plan mensuel",
+  "subscribe_cta_annual": "Commencer le plan annuel",
+  "subscribe_already_pro": "Vous avez déjà un abonnement Pro.",
+  "subscribe_manage": "Gérer l'abonnement",
+  "subscribe_benefit_audio": "Chapitres audio illimités par jour",
+  "subscribe_benefit_voices": "Sélection de voix pour tous les moteurs TTS",
+  "subscribe_benefit_translation": "Lire en français, indonésien, portugais et russe",
+  "subscribe_benefit_downloads": "Télécharger des chapitres pour une écoute hors ligne",
+  "subscribe_login_prompt": "Connectez-vous pour vous abonner",
+  "subscribe_login_cta": "Se connecter",
 
   "user_currently_reading": "En cours de lecture",
   "user_library_count": "Bibliothèque ({n})",