feat(payments): payment-event dispatcher + the provider-return UX

Deliver payment_events to connected clients as an in-app wallet-refresh push: a
background dispatcher drains undispatched events and publishes a KindNotification
"payment" signal, marking each delivered; the client bumps a wallet-refresh
counter the open Wallet screen watches, re-fetching in place. A return-focus
refetch is the fallback. The Robokassa Success/Fail return now serves a
self-closing page (the payment opens in a separate window) so the customer drops
back into the live app instead of a cold start. Integration test for the event
drain/mark queue.

ui/src/screens/Wallet.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import { onMount } from 'svelte';
   import Modal from '../components/Modal.svelte';
-  import { handleError } from '../lib/app.svelte';
+  import { app, handleError } from '../lib/app.svelte';
   import { gateway } from '../lib/gateway';
   import { t, i18n, type MessageKey } from '../lib/i18n/index.svelte';
   import { executionContext, formatAmount, needsWebSpendWarning, type SpendContext } from '../lib/wallet';
@@ -39,7 +39,26 @@
       loading = false;
     }
   }
-  onMount(load);
+  onMount(() => {
+    void load();
+    // Fallback to the payment push: re-fetch when the tab regains focus, i.e. the user returned
+    // from the provider's payment window.
+    const onVisible = () => {
+      if (!document.hidden) void load();
+    };
+    document.addEventListener('visibilitychange', onVisible);
+    return () => document.removeEventListener('visibilitychange', onVisible);
+  });
+
+  // Main path: a payment-intake push bumps app.walletRefresh; re-fetch in place when it changes
+  // (a credit landed while the screen is open).
+  let seenRefresh = app.walletRefresh;
+  $effect(() => {
+    if (app.walletRefresh !== seenRefresh) {
+      seenRefresh = app.walletRefresh;
+      void load();
+    }
+  });
 
   function sourceLabel(source: string): string {
     return t(`wallet.source.${source}` as MessageKey);