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/App.vue

@@ -39,6 +39,7 @@
 
     <!-- Global "mesh device detected" setup flow (fires on any page) -->
     <MeshDeviceSetupModal />
+    <ExternalExplorerModal />
 
     <!-- Nudge to back up the Lightning seed once a wallet exists (any page) -->
     <LndSeedBackupPrompt />
@@ -95,6 +96,7 @@ import Screensaver from './components/Screensaver.vue'
 import HelpGuideModal from './components/HelpGuideModal.vue'
 import GlobalAudioPlayer from './components/GlobalAudioPlayer.vue'
 import MeshDeviceSetupModal from './components/mesh/MeshDeviceSetupModal.vue'
+import ExternalExplorerModal from './components/ExternalExplorerModal.vue'
 import LndSeedBackupPrompt from './components/LndSeedBackupPrompt.vue'
 import { useMeshStore } from './stores/mesh'