fix(ui): improve dark mode text contrast for form labels

Replace hardcoded Tailwind color classes with design system CSS variables
to improve text visibility in dark mode:

- text-gray-900 dark:text-gray-100 → text-foreground
- text-gray-500/600 dark:text-gray-400 → text-muted-foreground
- bg-white dark:bg-gray-800 → bg-background

src/components/providers/forms/shared/EndpointField.tsx

@@ -40,7 +40,7 @@ export function EndpointField({
           <button
             type="button"
             onClick={onManageClick}
-            className="flex items-center gap-1 text-xs text-gray-600 dark:text-gray-400 hover:text-gray-900 dark:hover:text-gray-100 transition-colors"
+            className="flex items-center gap-1 text-xs text-muted-foreground hover:text-foreground transition-colors"
           >
             <Zap className="h-3.5 w-3.5" />
             {manageButtonLabel || defaultManageLabel}