receive/invoice: build POST body from snapshot, don't mutate $derived

The merchant receive and invoice pages mutated the $derived `invoice`
proxy (setType/setAmount/setMemo) then POSTed it. Mutating a $derived
and JSON.stringify-ing it serializes unreliably, so a stale type
(on-chain when lightning was expected) and a stale/10x-shifted amount
could leak from a prior interaction into POST /invoice (funk POS bug).

Build the body as { ...$state.snapshot(invoice), ...changes } so the
intended type/amount/address_type reliably win; update() takes the
plain body. Front-end only.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

1 files changed, 12 insertions, 11 deletions

diff --git a/src/routes/(app)/invoice/[id]/+page.svelte b/src/routes/(app)/invoice/[id]/+page.svelte
index 29dd815d..a2a299bd 100644
--- a/src/routes/(app)/invoice/[id]/+page.svelte
+++ b/src/routes/(app)/invoice/[id]/+page.svelte
@@ -70,10 +70,10 @@
     if ($amountPrompt && !amount) toggleAmount();
   });
 
-  let update = async () => {
+  let update = async (body) => {
     try {
       ({ id } = await post(`/invoice`, {
-        invoice,
+        invoice: body,
         user: { username, currency },
       }));
 
@@ -94,10 +94,12 @@
     if (type === types.lightning && !amount && typeof newAmount === "undefined")
       goto(`/${username}/receive`, { invalidateAll: true, noScroll: true });
     else {
-      if (typeof newAmount !== "undefined") invoice.amount = newAmount;
-      invoice.address_type = address_type;
-      invoice.type = type;
-      await update();
+      // Build the POST body from a snapshot + explicit changes. Mutating the
+      // $derived `invoice` proxy and POSTing it serializes unreliably, leaking
+      // stale type/amount/hash from a prior interaction (see funk POS bug).
+      const body = { ...$state.snapshot(invoice), type, address_type };
+      if (typeof newAmount !== "undefined") body.amount = newAmount;
+      await update(body);
     }
     settingType = false;
   };
@@ -113,10 +115,9 @@
     settingAmount = false;
 
     if (typeof $amountPrompt === "undefined") $amountPrompt = true;
-    invoice.amount = newAmount;
-    invoice.tip = 0;
+    const body = { ...$state.snapshot(invoice), amount: newAmount, tip: 0 };
 
-    await update();
+    await update(body);
   };
 
   let setMemo = async (e) => {
@@ -124,8 +125,8 @@
     e.stopPropagation();
 
     settingMemo = false;
-    invoice.memo = memo;
-    await update();
+    const body = { ...$state.snapshot(invoice), memo };
+    await update(body);
   };
 
   let settingMemo = $state();