fix(demo): relative asset paths in bitcoin-ui/fedimint-ui shells

The shells are served at / in their containers but under /app/<id>/ in
the public demo, where absolute /tailwind.css and /assets/... 404 — the
Bitcoin UI rendered unstyled with a giant unconstrained SVG. Relative
paths resolve correctly in both contexts.

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

docker/fedimint-ui/index.html

@@ -386,7 +386,7 @@
       <section class="glass-card">
         <div class="header">
           <div class="logo-gradient-border">
-            <img src="/assets/img/app-icons/fedimint.jpg" alt="Fedimint Guardian">
+            <img src="assets/img/app-icons/fedimint.jpg" alt="Fedimint Guardian">
           </div>
           <div class="title">
             <h1>Fedimint Guardian</h1>