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/ApiKeyInput.tsx

@@ -31,14 +31,14 @@ const ApiKeyInput: React.FC<ApiKeyInputProps> = ({
   const inputClass = `w-full px-3 py-2 pr-10 border rounded-lg text-sm transition-colors ${
     disabled
       ? "bg-muted border-border-default text-muted-foreground cursor-not-allowed"
-      : "border-border-default dark:bg-gray-800 dark:text-gray-100 focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20"
+      : "border-border-default bg-background text-foreground focus:outline-none focus:ring-2 focus:ring-blue-500/20 dark:focus:ring-blue-400/20"
   }`;
 
   return (
     <div className="space-y-2">
       <label
         htmlFor={id}
-        className="block text-sm font-medium text-gray-900 dark:text-gray-100"
+        className="block text-sm font-medium text-foreground"
       >
         {label} {required && "*"}
       </label>
@@ -58,7 +58,7 @@ const ApiKeyInput: React.FC<ApiKeyInputProps> = ({
           <button
             type="button"
             onClick={toggleShowKey}
-            className="absolute inset-y-0 right-0 flex items-center pr-3 text-gray-500 dark:text-gray-400 hover:text-gray-900 dark:hover:text-gray-100 transition-colors"
+            className="absolute inset-y-0 right-0 flex items-center pr-3 text-muted-foreground hover:text-foreground transition-colors"
             aria-label={showKey ? t("apiKeyInput.hide") : t("apiKeyInput.show")}
           >
             {showKey ? <EyeOff size={16} /> : <Eye size={16} />}

src/components/providers/forms/CodexConfigSections.tsx

@@ -47,7 +47,7 @@ export const CodexAuthSection: React.FC<CodexAuthSectionProps> = ({
     <div className="space-y-2">
       <label
         htmlFor="codexAuth"
-        className="block text-sm font-medium text-gray-900 dark:text-gray-100"
+        className="block text-sm font-medium text-foreground"
       >
         {t("codexConfig.authJson")}
       </label>
@@ -67,7 +67,7 @@ export const CodexAuthSection: React.FC<CodexAuthSectionProps> = ({
       )}
 
       {!error && (
-        <p className="text-xs text-gray-500 dark:text-gray-400">
+        <p className="text-xs text-muted-foreground">
           {t("codexConfig.authJsonHint")}
         </p>
       )}
@@ -120,12 +120,12 @@ export const CodexConfigSection: React.FC<CodexConfigSectionProps> = ({
       <div className="flex items-center justify-between">
         <label
           htmlFor="codexConfig"
-          className="block text-sm font-medium text-gray-900 dark:text-gray-100"
+          className="block text-sm font-medium text-foreground"
         >
           {t("codexConfig.configToml")}
         </label>
 
-        <label className="inline-flex items-center gap-2 text-sm text-gray-500 dark:text-gray-400 cursor-pointer">
+        <label className="inline-flex items-center gap-2 text-sm text-muted-foreground cursor-pointer">
           <input
             type="checkbox"
             checked={useCommonConfig}
@@ -167,7 +167,7 @@ export const CodexConfigSection: React.FC<CodexConfigSectionProps> = ({
       )}
 
       {!configError && (
-        <p className="text-xs text-gray-500 dark:text-gray-400">
+        <p className="text-xs text-muted-foreground">
           {t("codexConfig.configTomlHint")}
         </p>
       )}

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",
             })}

src/components/providers/forms/GeminiConfigSections.tsx

@@ -47,7 +47,7 @@ export const GeminiEnvSection: React.FC<GeminiEnvSectionProps> = ({
     <div className="space-y-2">
       <label
         htmlFor="geminiEnv"
-        className="block text-sm font-medium text-gray-900 dark:text-gray-100"
+        className="block text-sm font-medium text-foreground"
       >
         {t("geminiConfig.envFile", { defaultValue: "环境变量 (.env)" })}
       </label>
@@ -69,7 +69,7 @@ GEMINI_MODEL=gemini-3-pro-preview`}
       )}
 
       {!error && (
-        <p className="text-xs text-gray-500 dark:text-gray-400">
+        <p className="text-xs text-muted-foreground">
           {t("geminiConfig.envFileHint", {
             defaultValue: "使用 .env 格式配置 Gemini 环境变量",
           })}
@@ -124,14 +124,14 @@ export const GeminiConfigSection: React.FC<GeminiConfigSectionProps> = ({
       <div className="flex items-center justify-between">
         <label
           htmlFor="geminiConfig"
-          className="block text-sm font-medium text-gray-900 dark:text-gray-100"
+          className="block text-sm font-medium text-foreground"
         >
           {t("geminiConfig.configJson", {
             defaultValue: "配置文件 (config.json)",
           })}
         </label>
 
-        <label className="inline-flex items-center gap-2 text-sm text-gray-500 dark:text-gray-400 cursor-pointer">
+        <label className="inline-flex items-center gap-2 text-sm text-muted-foreground cursor-pointer">
           <input
             type="checkbox"
             checked={useCommonConfig}
@@ -180,7 +180,7 @@ export const GeminiConfigSection: React.FC<GeminiConfigSectionProps> = ({
       )}
 
       {!configError && (
-        <p className="text-xs text-gray-500 dark:text-gray-400">
+        <p className="text-xs text-muted-foreground">
           {t("geminiConfig.configJsonHint", {
             defaultValue: "使用 JSON 格式配置 Gemini 扩展参数（可选）",
           })}

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}