feat: on-chain handoff from the custom-zap dialog

Adds a "Send on-chain instead" link button at the bottom of
ZapCustomDialog. Tapping it opens OnchainZapSendDialog with the entered
amount and message prefilled, the note as zappedEvent, and the same
split-detection / dust-preview / fee-tier picker that the Reactions
zap chip uses.

This also fixes the participant zap path in nests audio rooms
(ParticipantHostActionsSheet long-press → "Zap") since that menu item
opens ZapCustomDialog under the hood — fixing the custom dialog covers
both entry points transitively.

OnchainZapSendDialog gains a `prefillComment: String = ""` parameter so
the message field carries over from the LN dialog.

Poll-note zaps (FilteredZapAmountChoicePopup) intentionally stay
Lightning-only — the poll_option vote is encoded in the kind:9734 zap
request and counted via kind:9735 receipts; kind:8333 on-chain
receipts have no poll_option analog and the count infrastructure
doesn't index them. This is a protocol design constraint, not a UI
oversight.

Coverage audit summary — every user-facing zap initiation point now
has on-chain support except the poll-voting path:
- ZapReaction (reactions row) ✅
- ZapAmountChoicePopup (popup chips) ✅
- ZapCustomDialog (custom amount + message) ✅ (this commit)
- ReusableZapButton (Zap the Devs + DVM buttons) ✅
- NestActionBar (audio room) ✅
- ParticipantHostActionsSheet (audio room participant) ✅ (transitive)
- ChatMessageCompose / live-activity headers (use ZapReaction) ✅
- FilteredZapAmountChoicePopup (poll votes) ⚠️ LN-only by protocol

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

@@ -48,6 +48,7 @@ import androidx.compose.material3.Scaffold
 import androidx.compose.material3.SuggestionChip
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
@@ -81,6 +82,7 @@ import com.vitorpamplona.amethyst.ui.navigation.navs.INav
 import com.vitorpamplona.amethyst.ui.navigation.topbars.TopBarWithBackButton
 import com.vitorpamplona.amethyst.ui.note.buttons.CloseButton
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.wallet.OnchainZapSendDialog
 import com.vitorpamplona.amethyst.ui.stringRes
 import com.vitorpamplona.amethyst.ui.theme.ButtonBorder
 import com.vitorpamplona.amethyst.ui.theme.DoubleHorzSpacer
@@ -89,6 +91,7 @@ import com.vitorpamplona.amethyst.ui.theme.Size55dp
 import com.vitorpamplona.amethyst.ui.theme.StdHorzSpacer
 import com.vitorpamplona.amethyst.ui.theme.ZeroPadding
 import com.vitorpamplona.amethyst.ui.theme.placeholderText
+import com.vitorpamplona.quartz.nip01Core.core.Event
 import com.vitorpamplona.quartz.nip57Zaps.LnZapEvent
 import kotlinx.collections.immutable.ImmutableList
 import kotlinx.coroutines.CancellationException
@@ -157,6 +160,11 @@ fun ZapCustomDialog(
 
     val presetAmounts = remember(accountViewModel) { accountViewModel.zapAmountChoices() }
 
+    // True while the on-chain hand-off sheet is open. Dismissing the on-chain
+    // sheet closes the whole zap flow — the user has already committed to a
+    // payment method, no "go back to Lightning" is offered here.
+    var sendOnchain by remember { mutableStateOf(false) }
+
     Dialog(
         onDismissRequest = { onClose() },
         properties =
@@ -326,9 +334,38 @@ fun ZapCustomDialog(
                     )
                     onClose()
                 }
+
+                // Hand off to the on-chain dialog with the entered amount +
+                // message prefilled. Disabled while the amount is empty so the
+                // user can't open a sheet that immediately gates on its own
+                // empty field.
+                TextButton(
+                    onClick = { sendOnchain = true },
+                    enabled = postViewModel.canSend() && !baseNote.isDraft(),
+                    modifier =
+                        Modifier
+                            .fillMaxWidth()
+                            .padding(top = 4.dp),
+                ) {
+                    Text(text = stringRes(id = R.string.send_onchain_instead))
+                }
             }
         }
     }
+
+    if (sendOnchain) {
+        OnchainZapSendDialog(
+            accountViewModel = accountViewModel,
+            onDismiss = {
+                sendOnchain = false
+                onClose()
+            },
+            recipientPubKey = baseNote.author?.pubkeyHex,
+            zappedEvent = baseNote.toEventHint<Event>(),
+            prefillAmountSats = postViewModel.value(),
+            prefillComment = postViewModel.customMessage.text,
+        )
+    }
 }
 
 @Composable

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

@@ -137,6 +137,7 @@ fun OnchainZapSendDialog(
     recipientPubKey: HexKey? = null,
     zappedEvent: EventHintBundle<out Event>? = null,
     prefillAmountSats: Long? = null,
+    prefillComment: String = "",
 ) {
     val sheetState = rememberModalBottomSheetState(skipPartiallyExpanded = true)
     val scope = rememberCoroutineScope()
@@ -152,7 +153,7 @@ fun OnchainZapSendDialog(
     var searchInput by remember { mutableStateOf("") }
     var selectedUser by remember { mutableStateOf<User?>(null) }
     var amountInput by remember { mutableStateOf(prefillAmountSats?.toString().orEmpty()) }
-    var comment by remember { mutableStateOf("") }
+    var comment by remember { mutableStateOf(prefillComment) }
     var feeTier by remember { mutableStateOf(FeeTier.NORMAL) }
     var fees by remember { mutableStateOf<FeeEstimates?>(null) }
 

amethyst/src/main/res/values/strings.xml

@@ -824,6 +824,7 @@
     <string name="quick_zap_amounts_onchain">Quick On-chain Zap Amounts</string>
     <string name="quick_zap_amounts_onchain_explainer">On-chain zaps pay miner fees, so amounts are usually larger than Lightning. Tap an amount to remove it.</string>
     <string name="new_amount_in_sats_onchain">New on-chain amount in sats</string>
+    <string name="send_onchain_instead">Send on-chain instead</string>
     <string name="zap_privacy_section">Zap Privacy</string>
     <string name="zap_type_section_explainer">Controls how your identity is shown when you send a zap.</string>
     <string name="wallet_connect_connect_app">Connect Wallet</string>