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/PageLoader.tsx

@@ -5,7 +5,7 @@ export function PageLoader() {
     <div className="flex h-full items-center justify-center">
       <div className="flex flex-col items-center gap-4">
         <Spinner size="lg" />
-        <p className="text-sm text-[#848b9b]">Loading&hellip;</p>
+        <p className="text-sm text-muted-foreground">Loading&hellip;</p>
       </div>
     </div>
   )