feat(wallet): external tx-explorer fallback with consent + On-chain settings tab

Pruned nodes can't run the Mempool app, but tx links blindly opened it
anyway. Now: local app when running; otherwise an external explorer
(default tx1138.com) behind a one-time amber consent modal that spells
out what the other server's operator learns (tx of interest + IP) and
lets the user point at their own instance (placeholder mempool.guide).
Wallet Settings gains an On-chain tab (explorer URL + don't-warn toggle);
tabs renamed Cashu/Fedi so five fit in the row.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/src/components/TransactionsModal.vue

@@ -100,7 +100,7 @@
 import { ref, computed } from 'vue'
 import { useI18n } from 'vue-i18n'
 import BaseModal from '@/components/BaseModal.vue'
-import { useAppLauncherStore } from '@/stores/appLauncher'
+import { useTxExplorer } from '@/composables/useTxExplorer'
 
 interface WalletTransaction {
   tx_hash: string
@@ -167,10 +167,12 @@ function kindLabel(tx: WalletTransaction): string {
   return ''
 }
 
+const txExplorer = useTxExplorer()
 function openInMempool(tx: WalletTransaction) {
   if (!isOnchain(tx)) return
-  // Overlay the explorer above the current page — never navigate away.
-  useAppLauncherStore().openSession('mempool', { path: `/tx/${tx.tx_hash}` })
+  // Local Mempool app when running (overlaid above this modal); external
+  // explorer with consent otherwise (pruned nodes can't run Mempool).
+  txExplorer.openTx(tx.tx_hash)
 }
 
 function formatTxTime(timestamp: number): string {