fix(demo): /app proxy (fixes 404s), mempool iframe, LND UI, icons

- nginx-demo.conf + vite proxy now route every /app/<id>/ to the mock backend, so
  the per-app mock UIs and the generic "Not available in the demo" notice render
  (previously only /app/filebrowser was proxied → most apps 404'd).
- Mempool and IndeeHub now load in the in-app iframe (not a new tab).
- Add an LND Lightning mock UI (channels, balances, routing) with dummy data;
  lnd/thunderhub are demoable. Notice page reworded to "Not available in the demo".
- Fix missing icons: Bitcoin Core → bitcoin-core.png, Mempool → mempool.webp.
- Pre-install only Bitcoin Core (drop duplicate Bitcoin Knots; still installable).

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

neode-ui/src/composables/useDemoIntro.ts

@@ -53,7 +53,7 @@ export function clearDemoIntroSeen(): void {
 // external site). Everything else shows "No demo" on a disabled install button
 // and is not launchable.
 const DEMO_EXTERNAL_URLS: Record<string, string> = {
-  // Real, public sites that we open instead of mocking.
+  // Real, public sites loaded in the in-app iframe.
   indeedhub: 'https://indee.tx1138.com/',
   mempool: 'https://mempool.space/testnet',
   'mempool-web': 'https://mempool.space/testnet',
@@ -68,14 +68,21 @@ const DEMO_MOCK_UI: Record<string, string> = {
   electrs: '/app/electrumx/',
   electrumx: '/app/electrumx/',
   'archy-electrs-ui': '/app/electrumx/',
+  lnd: '/app/lnd/',
+  'lnd-ui': '/app/lnd/',
+  'archy-lnd-ui': '/app/lnd/',
+  thunderhub: '/app/lnd/',
   fedimint: '/app/fedimint/',
   fedimintd: '/app/fedimint/',
   filebrowser: '/app/filebrowser/',
 }
 
-/** Apps that open in a new tab (external real sites) rather than an iframe. */
-export function isDemoExternal(appId: string): boolean {
-  return appId === 'mempool' || appId === 'mempool-web'
+/**
+ * Whether a demo app opens in a new tab. Nothing does now — both real external
+ * sites (mempool.space, indee.tx1138.com) load in the in-app iframe.
+ */
+export function isDemoExternal(_appId: string): boolean {
+  return false
 }
 
 /** Can this app be launched/installed in the demo? */