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/env/EnvWarningBanner.tsx

@@ -198,7 +198,7 @@ export function EnvWarningBanner({
                             <p className="text-xs text-gray-600 dark:text-gray-400 mt-1 break-all">
                               {t("env.field.value")}: {conflict.varValue}
                             </p>
-                            <p className="text-xs text-gray-500 dark:text-gray-500 mt-1">
+                            <p className="text-xs text-muted-foreground mt-1">
                               {t("env.field.source")}:{" "}
                               {getSourceDescription(conflict)}
                             </p>