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/CodexFormFields.tsx

@@ -98,7 +98,7 @@ export function CodexFormFields({
         <div className="space-y-2">
           <label
             htmlFor="codexModelName"
-            className="block text-sm font-medium text-gray-900 dark:text-gray-100"
+            className="block text-sm font-medium text-foreground"
           >
             {t("codexConfig.modelName", { defaultValue: "模型名称" })}
           </label>
@@ -110,9 +110,9 @@ export function CodexFormFields({
             placeholder={t("codexConfig.modelNamePlaceholder", {
               defaultValue: "例如: gpt-5-codex",
             })}
-            className="w-full px-3 py-2 border border-border-default dark:bg-gray-800 dark:text-gray-100 rounded-lg text-sm focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 transition-colors"
+            className="w-full px-3 py-2 border border-border-default bg-background text-foreground rounded-lg text-sm focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20 transition-colors"
           />
-          <p className="text-xs text-gray-500 dark:text-gray-400">
+          <p className="text-xs text-muted-foreground">
             {t("codexConfig.modelNameHint", {
               defaultValue: "指定使用的模型，将自动更新到 config.toml 中",
             })}