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

resources/lang/en.json

@@ -942,8 +942,11 @@
     "title": "Store",
     "patterns": "Skins",
     "flags": "Flags",
+    "packs": "Packs",
     "no_flags": "No flags available. Check back later for new items.",
-    "no_skins": "No skins available. Check back later for new items."
+    "no_skins": "No skins available. Check back later for new items.",
+    "no_packs": "No packs available. Check back later for new items.",
+    "currency_pack_purchase_success": "Currency pack purchase successful!"
   },
   "territory_patterns": {
     "title": "Skins",
@@ -963,7 +966,9 @@
     "rare": "Rare",
     "epic": "Epic",
     "legendary": "Legendary",
-    "adfree": "ad-free for life!"
+    "adfree": "ad-free for life!",
+    "hard": "Plutonium",
+    "soft": "Caps"
   },
   "flag_input": {
     "title": "Select Flag",