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/Spinner.tsx

@@ -15,7 +15,7 @@ export function Spinner({ size = 'md', className }: SpinnerProps) {
   return (
     <div
       className={cn(
-        'animate-spin rounded-full border-[#1e2130] border-t-primary',
+        'animate-spin rounded-full border-border border-t-primary',
         SIZES[size],
         className
       )}