fix(auth): configure OpenClaw default model when saving provider

- Add setOpenClawDefaultModel() to write the correct model to
  ~/.openclaw/openclaw.json based on the selected provider
- Maps: openrouter -> openrouter/anthropic/claude-sonnet-4,
  anthropic -> anthropic/claude-sonnet-4, etc.
- Call setOpenClawDefaultModel on provider:save IPC handler
- Fixes "No API key found for provider anthropic" when user
  configured OpenRouter (wrong default model was being used)

electron/main/ipc-handlers.ts

@@ -21,7 +21,7 @@ import {
 } from '../utils/secure-storage';
 import { getOpenClawStatus } from '../utils/paths';
 import { getSetting } from '../utils/store';
-import { saveProviderKeyToOpenClaw } from '../utils/openclaw-auth';
+import { saveProviderKeyToOpenClaw, setOpenClawDefaultModel } from '../utils/openclaw-auth';
 
 /**
  * Register all IPC handlers
@@ -228,6 +228,13 @@ function registerProviderHandlers(): void {
         }
       }
       
+      // Set the default model in OpenClaw config based on provider type
+      try {
+        setOpenClawDefaultModel(config.type);
+      } catch (err) {
+        console.warn('Failed to set OpenClaw default model:', err);
+      }
+      
       return { success: true };
     } catch (error) {
       return { success: false, error: String(error) };