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/dashboard/PerformanceCards.tsx

@@ -74,14 +74,14 @@ export function PerformanceCards() {
           style={{ animationDelay: `${400 + i * 60}ms` }}
         >
           <div className="flex items-center justify-between mb-2">
-            <p className="font-sans text-xs text-[0.5625rem] uppercase tracking-[0.1em] text-[#848b9b]">
+            <p className="font-sans text-xs text-[0.5625rem] uppercase tracking-[0.1em] text-muted-foreground">
               {card.label}
             </p>
             <card.icon size={14} style={{ color: card.iconColor }} />
           </div>
           <p className={cn(
             'font-heading text-2xl font-extrabold tracking-tight',
-            card.highlight ? 'text-[#67e8f9]' : 'text-[#e2e5eb]'
+            card.highlight ? 'text-accent-text' : 'text-foreground'
           )}>
             {card.value}
           </p>