refactor: replace hardcoded hex values with Tailwind semantic tokens

3,200+ hardcoded color values replaced with CSS variable-backed
Tailwind classes (bg-card, text-foreground, border-border, etc.).
Enables light mode via CSS variable swap. Only syntax highlighting
colors and intentional one-offs remain hardcoded (~15 values).

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

frontend/src/components/common/BrandWordmark.tsx

@@ -9,7 +9,7 @@ export function BrandWordmark({ size = 'sm', className }: BrandWordmarkProps) {
   return (
     <span
       className={cn(
-        'font-heading font-bold tracking-tight text-[#f0f2f5]',
+        'font-heading font-bold tracking-tight text-text-heading',
         size === 'sm' ? 'text-xl' : 'text-3xl',
         className
       )}