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/screens/Wallet.svelte

@@ -6,6 +6,7 @@
   import { t, i18n, type MessageKey } from '../lib/i18n/index.svelte';
   import { executionContext, formatAmount, needsWebSpendWarning, type SpendContext } from '../lib/wallet';
   import { isGooglePlayBuild } from '../lib/distribution';
+  import { openExternalUrl } from '../lib/links';
   import type { Wallet, Catalog, CatalogProduct } from '../lib/model';
 
   // The Wallet section: the context-visible chip balances, the active benefits, and the storefront.
@@ -78,6 +79,22 @@
       busy = false;
     }
   }
+
+  // onOrder funds a chip pack with money: it opens a pending order and sends the player to the
+  // provider's hosted-payment page. The chips are credited later, by the verified server callback;
+  // paying accepts the public offer (linked below the packs).
+  async function onOrder(p: CatalogProduct) {
+    if (busy) return;
+    busy = true;
+    try {
+      const order = await gateway.walletOrder(p.productId);
+      openExternalUrl(order.redirectUrl);
+    } catch (e) {
+      handleError(e);
+    } finally {
+      busy = false;
+    }
+  }
 </script>
 
 <div class="wallet" data-testid="wallet">
@@ -120,9 +137,14 @@
         <div class="row product" data-testid="product" data-kind="pack" data-pid={p.productId}>
           <span class="name">{p.title}</span>
           <span class="price">{formatAmount(p.moneyAmount, p.moneyCurrency)}&nbsp;{currencyLabel(p.moneyCurrency)}</span>
-          <button class="buy" disabled>{t('wallet.soon')}</button>
+          <button class="buy" data-testid="buy-pack" disabled={busy} onclick={() => onOrder(p)}>{t('wallet.buy')}</button>
         </div>
       {/each}
+      {#if packs.length > 0}
+        <p class="offer" data-testid="offer">
+          <a href="/offer/" target="_blank" rel="noopener noreferrer">{t('wallet.offer')}</a>
+        </p>
+      {/if}
     {/if}
 
     {#if !loading && values.length === 0 && (gpBuild || packs.length === 0)}
@@ -207,6 +229,14 @@
     border: 1px dashed var(--border);
     border-radius: var(--radius-sm);
   }
+  .offer {
+    margin: 2px 0 0;
+    text-align: center;
+    font-size: 0.85rem;
+  }
+  .offer a {
+    color: var(--text-muted);
+  }
   .warn-body {
     margin: 0 0 14px;
   }