feat(ui): wire the chip-pack purchase to Robokassa

Replace the disabled "Soon" pack action with a real purchase: the Wallet opens a
money order (wallet.order) and sends the player to the provider's hosted-payment
page (window.open via openExternalUrl); the chips are credited later by the
verified server callback. Add a public-offer link under the packs (paying
accepts the offer). Codec order round-trip unit test + a mock-e2e purchase test;
the Google Play stub and the chip-spend paths are unchanged.

ui/src/lib/mock/client.ts

@@ -41,6 +41,7 @@ import type {
   Variant,
   WordCheckResult,
   Wallet,
+  WalletOrder,
   Catalog,
 } from '../model';
 import { valueForLetter } from '../alphabet';
@@ -225,6 +226,14 @@ export class MockGateway implements GatewayClient {
     }
     return this.cloneWallet();
   }
+
+  async walletOrder(productId: string): Promise<WalletOrder> {
+    const p = this.mockCatalog.products.find((x) => x.productId === productId);
+    if (!p || p.kind !== 'pack') throw new GatewayError('not_a_pack');
+    // No real provider in mock: return a stub launch URL so the e2e can assert the purchase reaches
+    // the provider hand-off without leaving the app.
+    return { orderId: 'mock-order-' + productId, redirectUrl: 'https://mock.local/robokassa?order=' + productId };
+  }
   private cloneWallet(): Wallet {
     return {
       segments: this.mockWallet.segments.map((s) => ({ ...s })),