Buy flow: survive mobile app-switch fetch drops

Mobile drops in-flight fetches when the page returns from the wallet app, which
aborted the purchase before the tx ("Failed to fetch"). Retry /api/me and
/api/sponsor, and make waitTx keep polling through transient fetch failures.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/assets/wallet.js

@@ -109,9 +109,11 @@ window.IAPWallet = (function () {
   }
 
   async function waitTx(hash) {
-    for (let i = 0; i < 60; i++) {
-      const r = await (await fetch('/api/tx/' + hash)).json();
-      if (r.found) return { status: r.status, blockNumber: r.blockNumber };
+    for (let i = 0; i < 90; i++) {
+      try {
+        const r = await (await fetch('/api/tx/' + hash)).json();
+        if (r.found) return { status: r.status, blockNumber: r.blockNumber };
+      } catch (e) { /* transient fetch failure (e.g. mobile app-switch) — keep polling */ }
       await new Promise(res => setTimeout(res, 2500));
     }
     throw new Error('Timed out waiting for the transaction. Check the explorer.');