feat(subscriptions): add subscription plan UI and localization

Adds locale strings for subscription plan types (monthly/yearly/lifetime), pricing display, payment buttons, and success/empty states. Updates subscribe page title to include repository name for better context.

options/locale/locale_en-US.json

@@ -4297,6 +4297,16 @@
   "repo.subscribe.payment_required": "A subscription is required to view this repository's source code.",
   "repo.subscribe.button": "Subscribe for Access",
   "repo.subscribe.active": "You have an active subscription",
+  "repo.subscribe.monthly": "Monthly",
+  "repo.subscribe.yearly": "Yearly",
+  "repo.subscribe.lifetime": "Lifetime",
+  "repo.subscribe.per_month": "month",
+  "repo.subscribe.per_year": "year",
+  "repo.subscribe.one_time": "one-time payment",
+  "repo.subscribe.choose": "Choose Plan",
+  "repo.subscribe.pay_with_stripe": "Pay with Card",
+  "repo.subscribe.no_products": "No subscription plans are currently available for this repository.",
+  "repo.subscribe.success": "Thank you! Your subscription is now active.",
   "repo.cross_promoted": "Also Check Out",
   "repo.settings.license": "License",
   "repo.settings.license_type": "License Type",

templates/repo/subscribe.tmpl

@@ -3,7 +3,7 @@
 	{{template "repo/header" .}}
 	<div class="ui container">
 		<div class="ui segment">
-			<h2>{{svg "octicon-lock" 24}} {{ctx.Locale.Tr "repo.subscribe.title"}}</h2>
+			<h2>{{svg "octicon-lock" 24}} {{ctx.Locale.Tr "repo.subscribe.title" .Repository.FullName}}</h2>
 			<p>{{ctx.Locale.Tr "repo.subscribe.description"}}</p>
 
 			{{if .Products}}