fix(ui): improve text visibility in dark mode

Replace hardcoded gray color classes with semantic color classes
to fix poor text contrast in dark mode:

- MCP panel: server names, descriptions, tags, app labels
- Prompt panel: prompt names, descriptions, empty states
- Usage footer: timestamps, refresh buttons
- Update badge: close button icon
- API key input: disabled state text
- Env warning banner: source info text

Changes:
- `text-gray-400 dark:text-gray-500` → `text-muted-foreground`
  (fixes reversed dark mode logic)
- `text-gray-500 dark:text-gray-400` → `text-muted-foreground`
- `bg-gray-100 dark:bg-gray-800` → `bg-muted`

src/components/UpdateBadge.tsx

@@ -56,7 +56,7 @@ export function UpdateBadge({ className = "", onClick }: UpdateBadgeProps) {
         "
         aria-label={t("common.close")}
       >
-        <X className="w-3 h-3 text-gray-400 dark:text-gray-500" />
+        <X className="w-3 h-3 text-muted-foreground" />
       </button>
     </div>
   );