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/RouteError.tsx

@@ -55,12 +55,12 @@ export function RouteError() {
   }
 
   return (
-    <div className="flex min-h-screen flex-col items-center justify-center bg-[#0c0d10] p-8">
+    <div className="flex min-h-screen flex-col items-center justify-center bg-background p-8">
       <div className="max-w-md text-center">
-        <h1 className="mb-2 text-4xl font-bold text-[#e2e5eb]">Oops!</h1>
+        <h1 className="mb-2 text-4xl font-bold text-foreground">Oops!</h1>
         <h2 className="mb-2 text-xl font-semibold text-red-400">{errorMessage}</h2>
         {errorDetails && (
-          <p className="mb-4 text-[#848b9b]">{errorDetails}</p>
+          <p className="mb-4 text-muted-foreground">{errorDetails}</p>
         )}
         <div className="flex justify-center gap-4">
           <Button variant="secondary" onClick={() => navigate(-1)}>