Show bonus amount on currency packs (#3907)

Show bonus amount on currency packs

- Add `bonusAmount` field to `PackSchema` (non-negative int)
- Render a rotated green corner ribbon (`+X FREE!`) on pack tiles when
`bonusAmount > 0`
- Add `cosmetics.free` translation key with `numFree` param

<img width="720" height="359" alt="Screenshot 2026-05-12 at 7 40 12 AM"
src="https://github.com/user-attachments/assets/3dd70fc4-c922-47f4-aee6-055047b58563"
/>

Describe the PR.

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

regression is found:

evan

src/core/CosmeticSchemas.ts

@@ -89,6 +89,7 @@ export const PackSchema = CosmeticSchema.extend({
   displayName: z.string(),
   currency: z.enum(["hard", "soft"]),
   amount: z.number().int().positive(),
+  bonusAmount: z.number().int().nonnegative(),
 });
 
 export const SubscriptionSchema = CosmeticSchema.extend({