support for purchasing currency packs (#3629)

## Description:

Adds a currency pack system to the store. Players can purchase packs of
in-game currency (Plutonium and Caps) via Stripe checkout.

What's new:

* Pack schema (PackSchema) — new cosmetic type with currency
(hard/soft), amount, and displayName
* "Packs" tab in the Store — renders purchasable currency packs using
existing CosmeticButton infrastructure
* Stripe checkout flow — new createCurrencyPackCheckout API call and
handlePackPurchase handler
* Currency display in Account modal — shows Plutonium and Caps balances
when logged in
I* con components — <plutonium-icon> (animated green glow + rotate) and
<cap-icon> with new SVG assets
* Currency in UserMeResponse — player.currency.hard /
player.currency.soft added to the API schema

## Please complete the following:

- [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

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/client/Main.ts

@@ -636,6 +636,12 @@ class Client {
         return;
       }
 
+      const type = params.get("type");
+      if (type === "currency_pack") {
+        alertAndStrip(translateText("store.currency_pack_purchase_success"));
+        return;
+      }
+
       const cosmeticName = params.get("cosmetic");
       if (!cosmeticName) {
         alert("Something went wrong. Please contact support.");