refactor: simplify TOML common config handling by removing markers

- Remove COMMON_CONFIG_MARKER_START/END constants
- Simplify config snippet addition/removal logic
- Use natural append/replace approach instead of markers
- Fix unused variable warning
- Improve user experience with cleaner config output

src/components/ProviderForm/CodexConfigEditor.tsx

@@ -39,7 +39,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
   // 支持按下 ESC 关闭弹窗
   useEffect(() => {
     if (!isCommonConfigModalOpen) return;
-    
+
     const onKeyDown = (e: KeyboardEvent) => {
       if (e.key === "Escape") {
         e.preventDefault();
@@ -97,9 +97,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
           data-enable-grammarly="false"
         />
         {authError && (
-          <p className="text-xs text-red-500 dark:text-red-400">
-            {authError}
-          </p>
+          <p className="text-xs text-red-500 dark:text-red-400">{authError}</p>
         )}
         <p className="text-xs text-gray-500 dark:text-gray-400">
           Codex auth.json 配置内容
@@ -161,7 +159,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
       </div>
 
       {isCommonConfigModalOpen && (
-        <div 
+        <div
           className="fixed inset-0 z-50 flex items-center justify-center"
           onMouseDown={(e) => {
             if (e.target === e.currentTarget) closeModal();
@@ -169,7 +167,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
         >
           {/* Backdrop - 统一背景样式 */}
           <div className="absolute inset-0 bg-black/50 dark:bg-black/70 backdrop-blur-sm" />
-          
+
           {/* Modal - 统一窗口样式 */}
           <div className="relative bg-white dark:bg-gray-900 rounded-xl shadow-lg max-w-2xl w-full mx-4 max-h-[90vh] overflow-hidden flex flex-col">
             {/* Header - 统一标题栏样式 */}
@@ -186,7 +184,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
                 <X size={18} />
               </button>
             </div>
-            
+
             {/* Content - 统一内容区域样式 */}
             <div className="flex-1 overflow-auto p-6 space-y-4">
               <p className="text-sm text-gray-500 dark:text-gray-400">
@@ -194,7 +192,9 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
               </p>
               <textarea
                 value={commonConfigSnippet}
-                onChange={(e) => handleCommonConfigSnippetChange(e.target.value)}
+                onChange={(e) =>
+                  handleCommonConfigSnippetChange(e.target.value)
+                }
                 placeholder={`# Common Codex config
 # Add your common TOML configuration here`}
                 rows={12}
@@ -215,7 +215,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
                 </p>
               )}
             </div>
-            
+
             {/* Footer - 统一底部按钮样式 */}
             <div className="flex items-center justify-end gap-3 p-6 border-t border-gray-200 dark:border-gray-800 bg-gray-100 dark:bg-gray-800">
               <button