refactor(proxy): disable OpenRouter compat mode by default and hide UI toggle

OpenRouter now natively supports Claude Code compatible API (/v1/messages),
so format transformation (Anthropic ↔ OpenAI) is no longer needed by default.

- Change default value from `true` to `false` in both frontend and backend
- Hide the "OpenRouter Compatibility Mode" toggle in provider form
- Users can still enable it manually by adding `"openrouter_compat_mode": true` in config JSON
- Update unit tests to reflect new default behavior

src/components/providers/forms/ProviderForm.tsx

@@ -236,7 +236,7 @@ export function ProviderForm({
     } catch {
       // ignore
     }
-    return true;
+    return false; // OpenRouter now supports Claude Code compatible API, no need for transform
   }, [isOpenRouterProvider, settingsConfigValue]);
 
   const handleOpenRouterCompatChange = useCallback(
@@ -864,7 +864,7 @@ export function ProviderForm({
             defaultOpusModel={defaultOpusModel}
             onModelChange={handleModelChange}
             speedTestEndpoints={speedTestEndpoints}
-            showOpenRouterCompatToggle={isOpenRouterProvider}
+            showOpenRouterCompatToggle={false}
             openRouterCompatEnabled={openRouterCompatEnabled}
             onOpenRouterCompatChange={handleOpenRouterCompatChange}
           />