fix(zap): collapse split when creator == platform; close #3 audit

Finishes the remaining #3 zap audit items.

- zapSplit: when the platform fee would pay the SAME Lightning address as the
  creator (e.g. the creator is the platform operator), collapse to a single
  full-amount payment instead of sending two invoices to one wallet — avoids an
  extra routing fee and a confusing duplicate receipt for the same total.
- Audited split correctness: floor(amount*fee/100) with the creator taking the
  remainder loses no sats; a sub-1-sat fee rounds to 0 and the whole zap goes to
  the creator (no zero-amount platform send); fee is clamped 0–100 and a junk
  override falls back to the default. The earlier "fee landed in my own wallet"
  report is expected behavior (sender is the platform operator, or a self-zap),
  not a bug.
- Cheat-resistant tests: testCreatorEqualsPlatformCollapsesToOnePayment and
  testSplitRoundingPreservesEverySat in tests/zap-split.test.mjs.
- Receipt validation (9735) and NWC budget/retry UX confirmed already solid;
  audit plan + TODO #3 closed.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/payments/zapSplit.js

@@ -540,8 +540,8 @@ export async function splitAndZap(
   }
 
   const platformFee = clampPercent(getOverridePlatformFee());
-  const platformShare = Math.floor((amount * platformFee) / 100);
-  const creatorShare = amount - platformShare;
+  let platformShare = Math.floor((amount * platformFee) / 100);
+  let creatorShare = amount - platformShare;
 
   let platformAddress = null;
   if (platformShare > 0) {
@@ -551,6 +551,18 @@ export async function splitAndZap(
     if (!platformAddress) {
       throw new Error("Platform Lightning address is unavailable.");
     }
+
+    // When the platform fee would be paid to the very same Lightning address as
+    // the creator (e.g. the creator is the platform operator), splitting just
+    // sends two invoices to one wallet — extra routing fees and a confusing
+    // duplicate receipt for the same total. Collapse to a single full-amount
+    // payment to the creator; the recipient is unchanged and ends up with the
+    // same sats.
+    if (platformAddress.toLowerCase() === creatorAddress.toLowerCase()) {
+      creatorShare = amount;
+      platformShare = 0;
+      platformAddress = null;
+    }
   }
 
   const receipts = [];