fix(saas): remove hardcoded model fallback — dynamic from available models

- service.rs: template model passed as-is (Option<String>), no hardcoded fallback
- saas-types.ts: AgentConfigFromTemplate.model → string | null
- agentStore.ts: when model is null, resolve from saasStore.availableModels[0]
- AgentOnboardingWizard.tsx: restore full file (was corrupted), apply assignTemplate try/catch fix

desktop/src/lib/saas-types.ts

@@ -472,7 +472,7 @@ export interface AssignTemplateRequest {
  *  capabilities are merged into tools (no separate field). */
 export interface AgentConfigFromTemplate {
   name: string;
-  model: string;
+  model: string | null;
   system_prompt?: string;
   tools: string[];
   soul_content?: string;