fix(demo): never leak the release-server IP in the public demo

- Companion QR overlay: demo builds encode the demo's own origin
  (/packages/archipelago-companion.apk ships in the web image) instead of
  the vps2 raw URL.
- Dockerfile.web/.backend: build-time scrub replaces every remaining
  occurrence of the release-server address with registry.demo.internal and
  fails the build if any survives.
- Mock backend update-mirror list, sideload help text, and changelog prose
  no longer name the server address.
- Copy demo-images workflow into .gitea/workflows/ — Gitea ignores
  .github/workflows when .gitea/workflows exists, so the CI never ran.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

neode-ui/src/views/Apps.vue

@@ -348,7 +348,7 @@
 
           <div class="mt-5 rounded-xl border border-white/10 bg-white/[0.04] p-4 text-sm text-white/65">
             <p class="font-medium text-white/80 mb-2">Easy sources</p>
-            <p>Use images from Docker Hub, GHCR, the VPS2 Gitea registry (146.59.87.168:3000), or localhost. Good first candidates: Excalidraw, Stirling PDF, FreshRSS, Wallabag, HedgeDoc, CyberChef, Mealie, or PairDrop.</p>
+            <p>Use images from Docker Hub, GHCR, the Archipelago app registry, or localhost. Good first candidates: Excalidraw, Stirling PDF, FreshRSS, Wallabag, HedgeDoc, CyberChef, Mealie, or PairDrop.</p>
           </div>
 
           <div class="mt-5 flex gap-3">