refactor(endpoint): separate edit and create mode endpoint management (#192)

Optimize custom endpoint management logic to distinguish between edit and create modes:
- Edit mode: endpoints are read/written directly to backend via API
- Create mode: use draftCustomEndpoints to stage, save on submit
- Remove duplicate endpoint loading in useSpeedTestEndpoints
- Add isSaving state and initialCustomUrls tracking

src/components/providers/forms/CodexFormFields.tsx

@@ -24,7 +24,7 @@ interface CodexFormFieldsProps {
   onBaseUrlChange: (url: string) => void;
   isEndpointModalOpen: boolean;
   onEndpointModalToggle: (open: boolean) => void;
-  onCustomEndpointsChange: (endpoints: string[]) => void;
+  onCustomEndpointsChange?: (endpoints: string[]) => void;
 
   // Speed Test Endpoints
   speedTestEndpoints: EndpointCandidate[];