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/appSessionConfig.ts

@@ -78,9 +78,12 @@ export const IFRAME_BLOCKED_APPS = new Set<string>([])
 /** Resolve app URL using direct port mapping (source of truth) */
 export function resolveAppUrl(id: string, routeQueryPath?: string, runtimeUrl?: string): string {
   // Demo: route to the app's mock UI or real external site (mempool.space,
-  // indee.tx1138.com). Non-demoable apps fall through to a generic notice page.
+  // indee.tx1138.com). Carry through a deep-link path (e.g. /tx/<hash> for
+  // mempool). Non-demoable apps fall through to a generic notice page.
   if (IS_DEMO) {
-    return demoAppUrl(id) || `/app/${id}/`
+    const base = demoAppUrl(id)
+    if (base) return routeQueryPath ? base + routeQueryPath : base
+    return `/app/${id}/`
   }
 
   // External HTTPS apps