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/PageHeader.tsx

@@ -25,11 +25,11 @@ export function PageHeader({
       <div className="flex items-start gap-3">
         {icon && <div className="shrink-0">{icon}</div>}
         <div>
-          <h1 className={cn('text-2xl font-bold font-heading text-[#e2e5eb]', titleClassName)}>
+          <h1 className={cn('text-2xl font-bold font-heading text-foreground', titleClassName)}>
             {title}
           </h1>
           {description && (
-            <p className={cn('mt-1 text-sm text-[#848b9b]', descriptionClassName)}>
+            <p className={cn('mt-1 text-sm text-muted-foreground', descriptionClassName)}>
               {description}
             </p>
           )}