Create a default project when the first task has none

Why: On an empty workspace the primary button and the empty state both say to start by creating a task, but the action only toasted that a project was required.
Evidence: A fresh embedded instance returns an empty project list, so the first click never opened a conversation.
Impact: The first task now creates 「默认项目」 and opens it. Existing projects are unchanged.
Refs: walkthrough of the empty workspace on main
Co-authored-by: Cursor <cursoragent@cursor.com>

src/store.ts

@@ -186,10 +186,16 @@ export const store = {
     await store.refreshArtifacts(projectId)
   },
 
-  /** 新建任务（=会话）：默认当前项目上下文；显式 projectId 用于"新建于指定项目"。 */
+  /** 新建任务（=会话）：默认当前项目上下文；显式 projectId 用于"新建于指定项目"。
+   * 空库时主按钮与空状态都写着「点击新建任务开始」，这里先落一个默认项目再开会话。 */
   async createTask(projectId?: string) {
-    const pid = projectId ?? state.currentProjectId
-    if (!pid) { store.toast('请先创建项目'); return }
+    let pid = projectId ?? state.currentProjectId
+    if (!pid && state.projects.length > 0) pid = state.projects[0].id
+    if (!pid) {
+      const p = await store.createProject('默认项目')
+      pid = p.id
+    }
+    if (pid !== state.currentProjectId) await store.openProject(pid)
     const c = await api.createConversation(pid)
     await store.refreshTasks()
     await store.openTask(c.id)