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/en.json

@@ -366,6 +366,26 @@
   "profile_upgrade_monthly": "Monthly \u2014 $6 / mo",
   "profile_upgrade_annual": "Annual \u2014 $48 / yr",
   "profile_free_limits": "Free plan: 3 audio chapters per day, English reading only.",
+  "subscribe_page_title": "Go Pro \u2014 libnovel",
+  "subscribe_heading": "Read more. Listen more.",
+  "subscribe_subheading": "Upgrade to Pro and unlock the full libnovel experience.",
+  "subscribe_monthly_label": "Monthly",
+  "subscribe_monthly_price": "$6",
+  "subscribe_monthly_period": "per month",
+  "subscribe_annual_label": "Annual",
+  "subscribe_annual_price": "$48",
+  "subscribe_annual_period": "per year",
+  "subscribe_annual_save": "Save 33%",
+  "subscribe_cta_monthly": "Start monthly plan",
+  "subscribe_cta_annual": "Start annual plan",
+  "subscribe_already_pro": "You already have a Pro subscription.",
+  "subscribe_manage": "Manage subscription",
+  "subscribe_benefit_audio": "Unlimited audio chapters per day",
+  "subscribe_benefit_voices": "Voice selection across all TTS engines",
+  "subscribe_benefit_translation": "Read in French, Indonesian, Portuguese, and Russian",
+  "subscribe_benefit_downloads": "Download chapters for offline listening",
+  "subscribe_login_prompt": "Sign in to subscribe",
+  "subscribe_login_cta": "Sign in",
 
   "user_currently_reading": "Currently Reading",
   "user_library_count": "Library ({n})",