feat: add WebBackButton to all sub-pages, widen renew page layout

- New WebBackButton component: visible on web, hidden in Telegram
  (native back button handles navigation there)
- Added to: Subscription detail, SubscriptionPurchase, RenewSubscription
- RenewSubscription: removed max-w-lg, grid layout for period cards

src/components/WebBackButton.tsx

@@ -0,0 +1,32 @@
+import { Link } from 'react-router';
+import { usePlatform } from '../platform';
+import { BackIcon } from './icons';
+
+interface WebBackButtonProps {
+  to: string;
+  replace?: boolean;
+  className?: string;
+}
+
+/**
+ * Back button visible only on web platform.
+ * Hidden in Telegram Mini App — native back button handles navigation there.
+ */
+export function WebBackButton({ to, replace, className }: WebBackButtonProps) {
+  const { platform } = usePlatform();
+
+  if (platform === 'telegram') return null;
+
+  return (
+    <Link
+      to={to}
+      replace={replace}
+      className={
+        className ||
+        'flex h-10 w-10 items-center justify-center rounded-xl border border-dark-700 bg-dark-800 transition-colors hover:border-dark-600'
+      }
+    >
+      <BackIcon />
+    </Link>
+  );
+}