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/ui/Textarea.tsx

@@ -10,8 +10,8 @@ export function Textarea({ className, error, id, ...props }: TextareaProps) {
       <textarea
         id={id}
         className={cn(
-          'flex w-full rounded-md border border-[#1e2130] bg-[#14161d] px-3 py-2 text-sm text-[#e2e5eb]',
-          'placeholder:text-[#848b9b]',
+          'flex w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground',
+          'placeholder:text-muted-foreground',
           'focus:border-primary/30 focus:outline-hidden focus:ring-1 focus:ring-primary/20',
           'disabled:cursor-not-allowed disabled:opacity-50',
           error && 'border-red-400/50 focus:border-red-400 focus:ring-red-400/20',