fix: replace all remaining old brand tokens (text-brand-dark, border-brand-border, bg-white opacity)

30 references to removed CSS variables: border-brand-border → border-[#1e2130],
text-brand-text-muted → text-[#4f5666], text-brand-dark → text-white,
bg-white/[0.04] → bg-[#191c25], hover:border-white/[0.12] → hover:border-[#2a2f3d].

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/assistant/ChatSidebar.tsx

@@ -31,7 +31,7 @@ export function ChatSidebar({
       <div className="px-4 py-3 border-b shrink-0" style={{ borderColor: 'var(--glass-border)' }}>
         <button
           onClick={onNewChat}
-          className="w-full flex items-center justify-center gap-2 bg-[#22d3ee] text-brand-dark font-semibold text-sm rounded-lg px-4 py-2.5 hover:brightness-110 active:scale-[0.98] transition-all"
+          className="w-full flex items-center justify-center gap-2 bg-[#22d3ee] text-white font-semibold text-sm rounded-lg px-4 py-2.5 hover:brightness-110 active:scale-[0.98] transition-all"
         >
           <Plus size={16} />
           New Chat
@@ -103,7 +103,7 @@ function ChatItem({
         'group flex items-center gap-2 px-3 py-2.5 mx-1.5 rounded-lg cursor-pointer transition-colors',
         isActive
           ? 'bg-[rgba(34,211,238,0.10)] text-[#e2e5eb]'
-          : 'text-[#848b9b] hover:bg-white/[0.04] hover:text-[#e2e5eb]'
+          : 'text-[#848b9b] hover:bg-[#191c25] hover:text-[#e2e5eb]'
       )}
     >
       <MessageSquare size={14} className="shrink-0" />