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/WalletSettingsModal.vue

@@ -149,6 +149,41 @@
       </p>
     </div>
 
+    <!-- ===================== On-chain ===================== -->
+    <div v-show="activeTab === 'onchain'">
+      <p class="text-white/60 text-sm mb-4">
+        Where "view transaction" links open. When the Mempool app is installed and running
+        on this node, it is always used — private, no third parties. Without it (pruned
+        Bitcoin nodes can't run Mempool), links open on the explorer below.
+      </p>
+
+      <label class="block text-sm text-white/80 mb-1">Transaction explorer</label>
+      <input
+        v-model="explorerUrlInput"
+        :placeholder="EXPLORER_PLACEHOLDER"
+        spellcheck="false"
+        class="w-full rounded-lg bg-white/[0.06] border border-white/10 text-white px-3 py-2 text-sm font-mono focus:outline-none focus:border-orange-400/60"
+        @change="saveExplorer"
+      />
+      <p class="text-[11px] text-white/40 mt-1">
+        Any Mempool-compatible instance works. Default: tx1138.com.
+      </p>
+
+      <div class="mt-3 p-3 rounded-lg border border-amber-400/25 bg-amber-500/10 text-amber-200/80 text-xs leading-relaxed">
+        ⚠️ Opening a transaction on an external explorer tells that server's operator which
+        transaction you're interested in, plus your IP address. Use a server you trust.
+      </div>
+
+      <label class="flex items-center gap-2 mt-4 text-sm text-white/70 cursor-pointer select-none">
+        <input type="checkbox" v-model="explorerAcknowledged" class="accent-orange-400" @change="saveExplorer" />
+        Don't warn me each time before opening the external explorer
+      </label>
+
+      <div class="flex gap-3 mt-6">
+        <button @click="close" class="flex-1 glass-button px-4 py-2 rounded-lg text-sm">{{ t('common.close') }}</button>
+      </div>
+    </div>
+
     <!-- ===================== Ark ===================== -->
     <div v-show="activeTab === 'ark'">
       <div class="flex items-start gap-2 mb-4">
@@ -253,24 +288,38 @@ import { useI18n } from 'vue-i18n'
 import { rpcClient } from '@/api/rpc-client'
 import BaseModal from '@/components/BaseModal.vue'
 import LightningChannelsPanel from '@/components/LightningChannelsPanel.vue'
+import { useTxExplorer, EXPLORER_PLACEHOLDER } from '@/composables/useTxExplorer'
 
 const { t } = useI18n()
 
 const props = defineProps<{ show: boolean }>()
 const emit = defineEmits<{ close: []; changed: [] }>()
 
+// Short labels on purpose — five tabs share one row ("Fedi", not
+// "Fedimint Federations") so On-chain fits.
 const tabs = [
   { key: 'channels' as const, label: 'Channels' },
-  { key: 'cashu' as const, label: 'Cashu Mints' },
-  { key: 'fedimint' as const, label: 'Fedimint Federations' },
+  { key: 'cashu' as const, label: 'Cashu' },
+  { key: 'fedimint' as const, label: 'Fedi' },
   { key: 'ark' as const, label: 'Ark' },
+  { key: 'onchain' as const, label: 'On-chain' },
 ]
-const activeTab = ref<'channels' | 'cashu' | 'fedimint' | 'ark'>('channels')
+const activeTab = ref<'channels' | 'cashu' | 'fedimint' | 'ark' | 'onchain'>('channels')
 
 // Backed by wallet.fedimint-list / -join / -leave (fedimint-clientd HTTP bridge).
 // Join degrades gracefully with a clear error if the Fedimint client app isn't installed.
 const fedimintBackendReady = true
 
+// ---- On-chain: external transaction explorer ----
+const txExplorer = useTxExplorer()
+const explorerUrlInput = ref(txExplorer.prefs.value.url)
+const explorerAcknowledged = ref(txExplorer.prefs.value.acknowledged)
+function saveExplorer() {
+  txExplorer.setExplorer(explorerUrlInput.value, explorerAcknowledged.value)
+  // Reflect normalization (empty input falls back to the default).
+  explorerUrlInput.value = txExplorer.prefs.value.url
+}
+
 // ---- Cashu mints ----
 const mints = ref<string[]>([])
 const newMint = ref('')