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/editor-ai/EditorAIPanel.tsx

@@ -65,7 +65,7 @@ export function EditorAIPanel({
         </div>
         <button
           onClick={onClose}
-          className="p-1.5 rounded-lg hover:bg-[#1e2130] text-muted-foreground hover:text-foreground transition-colors"
+          className="p-1.5 rounded-lg hover:bg-border text-muted-foreground hover:text-foreground transition-colors"
         >
           <X size={16} />
         </button>