feat(ui): add merge preview to common config editor components

Add real-time merge preview functionality to all config editor components
(Claude, Codex, Gemini) that shows the final merged configuration when
common config is enabled.

UI changes:
- Add finalConfig/finalEnv prop to display merged result
- Add toggle button to show/hide merge preview (Eye/EyeOff icons)
- Auto-show preview when common config is enabled
- Display custom config editor with label when preview is visible
- Show read-only merged preview below custom config
- Add visual indicators: "只读" badge, green hint text

The preview helps users understand how their custom configuration
combines with the shared common config template before saving.

src/components/providers/forms/CodexConfigEditor.tsx

@@ -30,6 +30,9 @@ interface CodexConfigEditorProps {
   onExtract?: () => void;
 
   isExtracting?: boolean;
+
+  /** 最终合并后的配置（只读预览） */
+  finalConfig?: string;
 }
 
 const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
@@ -47,6 +50,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
   configError,
   onExtract,
   isExtracting,
+  finalConfig,
 }) => {
   const [isCommonConfigModalOpen, setIsCommonConfigModalOpen] = useState(false);
 
@@ -76,6 +80,7 @@ const CodexConfigEditor: React.FC<CodexConfigEditorProps> = ({
         onEditCommonConfig={() => setIsCommonConfigModalOpen(true)}
         commonConfigError={commonConfigError}
         configError={configError}
+        finalConfig={finalConfig}
       />
 
       {/* Common Config Modal */}