fix(cashu): address pre-merge audit findings

- Invalidate the cached NUT-13 seed in applyEvents whenever the live kind:17375
  changes, so after a P2PK key rotation (recreateNutzapKey, or a rotation from
  another client) deterministic secrets re-derive from the new key instead of a
  stale cached seed. Removes the now-redundant reset in recreateNutzapKey.
- AccountSettings.updateNutzapInfo no longer backs up a mints-less kind:10019
  (the "stop receiving nutzaps" tombstone), clearing the backup instead — so the
  empty event round-tripping back through LocalCache can't undo clearNutzapInfo()
  and resurrect a withdrawn nutzap advertisement on next launch.
- Key keyMode's remember on isEditMode in AddCashuWalletScreen so a wallet
  delivered after first composition flips to KeepCurrent, preventing a silent
  key rotation on save in the cold-open race.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXRAunSJS2dBx7B79qTMew

amethyst/src/main/java/com/vitorpamplona/amethyst/model/AccountSettings.kt

@@ -942,6 +942,14 @@ class AccountSettings(
 
     fun updateNutzapInfo(newNutzapInfo: NutzapInfoEvent?) {
         if (newNutzapInfo == null || newNutzapInfo.tags.isEmpty()) return
+        // A mints-less kind:10019 is the "stop receiving nutzaps" tombstone
+        // (an empty replacement carrying only an `alt` tag). Don't restore it
+        // on next launch — backing it up would undo clearNutzapInfo() once the
+        // empty event round-trips back through LocalCache.
+        if (newNutzapInfo.mints().isEmpty()) {
+            clearNutzapInfo()
+            return
+        }
         if (backupNutzapInfo?.id != newNutzapInfo.id) {
             backupNutzapInfo = newNutzapInfo
             saveAccountSettings()

amethyst/src/main/java/com/vitorpamplona/amethyst/model/nip60Cashu/CashuWalletState.kt

@@ -563,6 +563,11 @@ class CashuWalletState(
             // Any wallet event resolves the "discovering" state — whether it
             // came from cache backfill or a fresh relay delivery.
             _discovering.value = false
+            // The NUT-13 seed is derived from the wallet's P2PK key. A new
+            // kind:17375 may carry a rotated key (our own recreateNutzapKey, or
+            // a rotation from another client), so drop the cached seed and let
+            // ensureSeed re-derive from whatever key the live event now holds.
+            cachedSeed = null
             walletEventInternal?.let { evt ->
                 _mints.value =
                     runCatching { evt.mints(signer) }
@@ -838,10 +843,9 @@ class CashuWalletState(
             p2pkPrivkeyHex = manualPrivkeyHex?.takeIf { it.isNotBlank() },
             nutzapRelays = outboxRelaysFlow.value.toList(),
         )
-        // The NUT-13 seed is derived from the P2PK key, which just changed —
-        // drop the cache so the next mint op re-derives from the new key
-        // (re-populated lazily by ensureSeed once the new kind:17375 lands).
-        cachedSeed = null
+        // The NUT-13 seed (derived from the P2PK key) is invalidated by
+        // applyEvents when the new kind:17375 round-trips in, so it re-derives
+        // from the rotated key. No need to reset it here.
     }
 
     // ============================================================

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/wallet/AddCashuWalletScreen.kt

@@ -99,7 +99,11 @@ fun AddCashuWalletScreen(
     // suggestions on first open instead of waiting for the next relay
     // round-trip. Cheap (one sweep over the existing cache); idempotent.
     LaunchedEffect(Unit) { LocalCache.ensureMintDirectoryBackfilled() }
-    var keyMode by remember {
+    // Keyed on isEditMode so that if the wallet is delivered AFTER this screen
+    // first composes (existingWallet null → non-null), keyMode flips to
+    // KeepCurrent. Otherwise a cold open with an undelivered wallet would keep
+    // AutoGenerate and silently rotate the key on save, orphaning nutzaps.
+    var keyMode by remember(isEditMode) {
         mutableStateOf(
             if (isEditMode) CashuWalletViewModel.P2pkKeyMode.KeepCurrent else CashuWalletViewModel.P2pkKeyMode.AutoGenerate,
         )