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/components/CapIcon.ts

@@ -0,0 +1,29 @@
+import { html, LitElement } from "lit";
+import { customElement, property } from "lit/decorators.js";
+import { assetUrl } from "../../core/AssetUrls";
+
+@customElement("cap-icon")
+export class CapIcon extends LitElement {
+  @property({ type: Number })
+  size: number = 48;
+
+  createRenderRoot() {
+    return this;
+  }
+
+  render() {
+    return html`
+      <div
+        class="inline-flex items-center justify-center"
+        style="width:${this.size}px; height:${this.size}px;"
+      >
+        <img
+          src=${assetUrl("images/BottleCapIcon.svg")}
+          alt="Caps"
+          style="width:${this.size}px; height:${this.size}px;"
+          draggable="false"
+        />
+      </div>
+    `;
+  }
+}