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/ErrorBoundary.tsx

@@ -39,10 +39,10 @@ function DefaultFallback({ error, resetError }: FallbackProps) {
         <h2 className="mb-2 text-xl font-semibold text-red-400">
           Something went wrong
         </h2>
-        <p className="mb-4 text-[#848b9b]">
+        <p className="mb-4 text-muted-foreground">
           An unexpected error occurred. Please try refreshing the page.
         </p>
-        <pre className="mb-4 overflow-auto rounded-xl bg-white/5 border border-[#1e2130] p-3 text-left text-xs text-red-400">
+        <pre className="mb-4 overflow-auto rounded-xl bg-white/5 border border-border p-3 text-left text-xs text-red-400">
           {error.message}
         </pre>
         <div className="flex justify-center gap-3">