feat: 支持 uv 从 pyproject/requirements 创建虚拟环境

- 工作目录探测新增 pyproject.toml / requirements*.txt 扫描、uv 可用性与 can_create
- 新增 POST /api/v1/python/venv/create：已有虚拟环境优先复用；pyproject 走 uv sync，requirements 走 uv venv + uv pip install，并使用默认 PyPI 镜像
- 编辑器在缺少虚拟环境且具备依赖文件时显示「用 uv 创建环境」；没有 uv 或没有依赖文件时明确报错
- 解释器字段改为可选，留空由服务端默认 python.exe 并使用虚拟环境；Python 任务缺少虚拟环境/依赖文件时运行直接失败，不再回退系统解释器
- 补后端探测/创建测试、前端入口测试与 README

backend/src/tasks/service.rs

@@ -28,6 +28,7 @@ pub async fn create(pool: &SqlitePool, mut request: CreateTaskRequest) -> ApiRes
     request.name = request.name.trim().to_owned();
     request.cron = request.cron.trim().to_owned();
     request.timezone = request.timezone.trim().to_owned();
+    normalize_action(&mut request.action);
 
     validate_task_id(&request.id)?;
     validate_common(
@@ -57,6 +58,7 @@ pub async fn update(
     request.name = request.name.trim().to_owned();
     request.cron = request.cron.trim().to_owned();
     request.timezone = request.timezone.trim().to_owned();
+    normalize_action(&mut request.action);
 
     validate_common(
         &request.name,
@@ -180,6 +182,20 @@ fn validate_common(
     Ok(())
 }
 
+fn normalize_action(action: &mut TaskAction) {
+    if let TaskAction::Script {
+        venv, interpreter, ..
+    } = action
+    {
+        if interpreter.trim().is_empty() {
+            *interpreter = "python.exe".to_owned();
+        }
+        if venv.as_deref().is_some_and(|value| value.trim().is_empty()) {
+            *venv = None;
+        }
+    }
+}
+
 fn validate_action(action: &TaskAction) -> ApiResult<()> {
     match action {
         TaskAction::Script {