feat: Fix SEO agent behavior and add z.ai API validation

- Add "SEO-First" mode to prevent unwanted agent switching
- SEO agent now stays locked and answers queries through SEO lens
- Add [SUGGEST_AGENT:xxx] marker for smart agent suggestions
- Add suggestion banner UI with Switch/Dismiss buttons
- Prevent auto-switching mid-response

- Add validateConnection() method to ZaiPlanService
- Add debounced API key validation (500ms) in Settings
- Add inline status indicators (valid/validating/error)
- Add persistent validation cache (5min) in localStorage
- Add "Test Connection" button for manual re-validation
- Add clear error messages for auth failures

- Add ApiValidationStatus interface
- Add apiValidationStatus state for tracking connection states
- Add setApiValidationStatus action

- Real-time API key validation in Settings panel
- Visual status indicators (✓/✗/🔄)
- Agent suggestion banner with Switch/Dismiss actions

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

lib/services/model-adapter.ts

@@ -74,6 +74,14 @@ export class ModelAdapter {
     return this.qwenService.hasOAuthToken();
   }
 
+  async validateConnection(provider: ModelProvider): Promise<APIResponse<{ valid: boolean; models?: string[] }>> {
+    const service = this.getService(provider);
+    if (!service || !service.validateConnection) {
+      return { success: false, error: `Provider ${provider} does not support connection validation` };
+    }
+    return await service.validateConnection();
+  }
+
   private isProviderAuthenticated(provider: ModelProvider): boolean {
     switch (provider) {
       case "qwen":