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/pages/QuickStartPage.tsx

@@ -18,7 +18,7 @@ export function QuickStartPage() {
       <div className="p-6 space-y-5 max-w-5xl mx-auto">
         {/* Greeting */}
         <div className="fade-in" style={{ animationDelay: '100ms' }}>
-          <h1 className="font-heading text-3xl font-extrabold tracking-tight text-[#e2e5eb]">
+          <h1 className="font-heading text-3xl font-extrabold tracking-tight text-foreground">
             Good{' '}
             {new Date().getHours() < 12
               ? 'morning'
@@ -27,7 +27,7 @@ export function QuickStartPage() {
               : 'evening'}
             , {user?.name?.split(' ')[0] || 'there'}
           </h1>
-          <p className="mt-1 text-sm text-[#848b9b]">
+          <p className="mt-1 text-sm text-muted-foreground">
             {new Date().toLocaleDateString('en-US', {
               weekday: 'long',
               month: 'long',