feat(frontend): Discord-style dark redesign

- full design system rewrite (dark theme, system tokens)
- chat shell: server rail / channel sidebar / message area / members
- message rows with avatars, author + time; empty/loading states
- login screen redesign; add-guild moved to a modal
- removed debug v1/v2/v4 panel toggles; members toggle in topbar

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/layouts/AppLayout.tsx

@@ -1,17 +1,5 @@
-import { Link, Outlet } from 'react-router-dom'
-import { useAuth } from '../auth/auth-context'
+import { Outlet } from 'react-router-dom'
 
 export default function AppLayout() {
-  const { isAuthed } = useAuth()
-
-  return (
-    <main className="page-content">
-      {!isAuthed ? (
-        <nav className="row-wrap" style={{ marginBottom: 12 }}>
-          <Link to="/login">Login</Link>
-        </nav>
-      ) : null}
-      <Outlet />
-    </main>
-  )
+  return <Outlet />
 }