feat(demo): real testnet tx links + interactive buy-files flow

- Tx/explorer links open mempool.space/testnet/tx/<id>; the backend hydrates the
  wallet's transactions with REAL recent testnet txids at startup (best-effort,
  falls back to mock hashes offline). Mempool app + demo-external apps open in a
  new tab; deep-link paths are carried through.
- Add the content.* paid-download handlers the buy flow needs (owned-list,
  preview-peer, download-peer-{paid,invoice,onchain}, request-invoice,
  invoice-status, request-onchain, onchain-status) — every path resolves to a
  success state with testnet receive addresses / bolt11 invoices so visitors can
  walk the full buy → unlock journey.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

neode-ui/src/views/AppSession.vue

@@ -109,6 +109,7 @@ import { useAppIdentity } from './appSession/useAppIdentity'
 import { useNostrBridge } from './appSession/useNostrBridge'
 import { openExternalUrl } from '@/utils/openExternal'
 import { useElectrsSync } from '@/composables/useElectrsSync'
+import { IS_DEMO, isDemoExternal } from '@/composables/useDemoIntro'
 
 const props = defineProps<{
   appIdProp?: string
@@ -157,7 +158,11 @@ const packageEntry = computed(() => store.data?.['package-data']?.[appId.value]
 const blockedReason = computed(() => launchBlockedReason(appId.value, packageEntry.value))
 const blockedTitle = computed(() => appId.value === 'fedimint' || appId.value === 'fedimintd' ? 'Waiting for Bitcoin sync' : 'App not ready')
 const isMobile = typeof window !== 'undefined' && window.innerWidth < 768
-const mustOpenNewTab = computed(() => NEW_TAB_APPS.has(appId.value))
+// In the demo, apps backed by a real external site that blocks iframing
+// (mempool.space) open in a new tab rather than the in-app session frame.
+const mustOpenNewTab = computed(() =>
+  NEW_TAB_APPS.has(appId.value) || (IS_DEMO && isDemoExternal(appId.value))
+)
 
 // ElectrumX shows a sync screen before its real UI (the Electrum server only
 // serves clients once its index is built). Poll /electrs-status while this is