fix(dross): collapse shell to 2 columns; topbar ◆ summons Dross

Removing #rightrail left a dead 360px grid column that narrowed #main.
Shell grid is now sidebar+main; the topbar ◆ (was Toggle-companion-rail)
now dispatches dross-toggle to open/close the floating bubble. Remaining
.rail-* CSS + chrome.js toggleRail are dead no-ops (minor cleanup later).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/components/topbar.js

@@ -4,7 +4,7 @@
 import { el, mount, clear } from '../dom.js';
 import { navigate } from '../router.js';
 import { on } from '../state.js';
-import { toggleSidebar, toggleRail } from './chrome.js';
+import { toggleSidebar } from './chrome.js';
 import { api } from '../api.js';
 
 // Cluster health → topbar pill. Returns [status, label, title].
@@ -72,7 +72,7 @@ export function renderTopbar(root) {
     el('div', { class: 'topbar-spacer' }),
     clusterPill,
     bell,
-    el('button', { class: 'chrome-toggle', title: 'Toggle companion chat', onclick: toggleRail }, '◆'),
+    el('button', { class: 'chrome-toggle', title: 'Summon Dross', onclick: () => window.dispatchEvent(new CustomEvent('dross-toggle')) }, '◆'),
     el('button', { class: 'icon-btn', onclick: () => alert('Agent-switching ships post-Plan-2.') }, 'Owner')
   );