feat(dross): global floating bubble; retire the right rail

Adds dross_bubble.js — a fixed FAB orb that opens a draggable,
anchored panel wired to wireAgentChat. Mic button rendered but
disabled (Phase 2). Swaps renderRightrail call in app.js; removes
dead <aside id="rightrail"> from index.html. rightrail.js kept in
place (unused).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/index.html

@@ -39,7 +39,6 @@
     <header id="topbar"></header>
     <aside id="sidebar"></aside>
     <main id="main"></main>
-    <aside id="rightrail"></aside>
   </div>
   <div id="modal-root"></div>
   <script type="module" src="/app.js"></script>