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/id.json

@@ -366,6 +366,26 @@
   "profile_upgrade_monthly": "Bulanan — $6 / bln",
   "profile_upgrade_annual": "Tahunan — $48 / thn",
   "profile_free_limits": "Paket gratis: 3 bab audio per hari, hanya bahasa Inggris.",
+  "subscribe_page_title": "Jadi Pro \u2014 libnovel",
+  "subscribe_heading": "Baca lebih. Dengarkan lebih.",
+  "subscribe_subheading": "Tingkatkan ke Pro dan buka pengalaman libnovel sepenuhnya.",
+  "subscribe_monthly_label": "Bulanan",
+  "subscribe_monthly_price": "$6",
+  "subscribe_monthly_period": "per bulan",
+  "subscribe_annual_label": "Tahunan",
+  "subscribe_annual_price": "$48",
+  "subscribe_annual_period": "per tahun",
+  "subscribe_annual_save": "Hemat 33%",
+  "subscribe_cta_monthly": "Mulai paket bulanan",
+  "subscribe_cta_annual": "Mulai paket tahunan",
+  "subscribe_already_pro": "Anda sudah berlangganan Pro.",
+  "subscribe_manage": "Kelola langganan",
+  "subscribe_benefit_audio": "Bab audio tak terbatas per hari",
+  "subscribe_benefit_voices": "Pilihan suara untuk semua mesin TTS",
+  "subscribe_benefit_translation": "Baca dalam bahasa Prancis, Indonesia, Portugis, dan Rusia",
+  "subscribe_benefit_downloads": "Unduh bab untuk didengarkan secara offline",
+  "subscribe_login_prompt": "Masuk untuk berlangganan",
+  "subscribe_login_cta": "Masuk",
 
   "user_currently_reading": "Sedang Dibaca",
   "user_library_count": "Perpustakaan ({n})",