fix(zap settings): de-dupe preset amounts so drag-reorder + chip keys stay valid

addAmount() allowed adding the same amount twice; two preset chips then share the
same key(amount) (Compose duplicate-key hazard) and the drag-reorder's
indexOf(amount) resolves to the wrong chip. De-dupe on add.

https://claude.ai/code/session_01HNE2z7CSYZ2G8KwC5fziJn

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/UpdateZapAmountViewModel.kt

@@ -72,7 +72,10 @@ class UpdateZapAmountViewModel : ViewModel() {
 
     fun addAmount() {
         val newValue = nextAmount.text.trim().toLongOrNull()
-        if (newValue != null) {
+        // De-dupe: a repeated amount would give two preset chips the same key (a
+        // Compose duplicate-key hazard) and make the drag-reorder's indexOf()
+        // resolve to the wrong chip.
+        if (newValue != null && newValue !in amountSet) {
             amountSet = amountSet + newValue
         }