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

contracts/openapi.json

@@ -383,6 +383,46 @@
         }
       }
     },
+    "/api/v1/python/venv/create": {
+      "post": {
+        "tags": [
+          "system"
+        ],
+        "operationId": "create_python_venv",
+        "requestBody": {
+          "content": {
+            "application/json": {
+              "schema": {
+                "$ref": "#/components/schemas/CreatePythonVenvRequest"
+              }
+            }
+          },
+          "required": true
+        },
+        "responses": {
+          "200": {
+            "description": "Created or reused project virtual environment",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/VenvProbeResponse"
+                }
+              }
+            }
+          },
+          "400": {
+            "description": "Project cannot create a virtual environment",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ErrorResponse"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/runs/{run_id}": {
       "get": {
         "tags": [
@@ -1155,6 +1195,19 @@
         },
         "additionalProperties": false
       },
+      "CreatePythonVenvRequest": {
+        "type": "object",
+        "description": "用 uv 为项目创建 `.venv`（pyproject 用 uv sync，requirements 用 uv venv + uv pip install）。",
+        "required": [
+          "working_dir"
+        ],
+        "properties": {
+          "working_dir": {
+            "type": "string"
+          }
+        },
+        "additionalProperties": false
+      },
       "CreateTaskRequest": {
         "type": "object",
         "required": [
@@ -1959,9 +2012,18 @@
         "required": [
           "status",
           "interpreter_kind",
-          "message"
+          "message",
+          "uv_available",
+          "has_pyproject",
+          "has_requirements",
+          "requirements_files",
+          "can_create"
         ],
         "properties": {
+          "can_create": {
+            "type": "boolean",
+            "description": "没有虚拟环境时，是否具备用 uv 创建的条件"
+          },
           "display_path": {
             "type": [
               "string",
@@ -1969,12 +2031,27 @@
             ],
             "description": "供界面展示的解释器路径；位于工作目录内时使用相对路径"
           },
+          "has_pyproject": {
+            "type": "boolean",
+            "description": "工作目录根部是否存在 pyproject.toml"
+          },
+          "has_requirements": {
+            "type": "boolean",
+            "description": "工作目录根部是否存在 requirements*.txt"
+          },
           "interpreter_kind": {
             "$ref": "#/components/schemas/InterpreterKind"
           },
           "message": {
             "type": "string"
           },
+          "requirements_files": {
+            "type": "array",
+            "items": {
+              "type": "string"
+            },
+            "description": "探测到的 requirements 文件名（仅根目录）"
+          },
           "source": {
             "oneOf": [
               {
@@ -1988,6 +2065,10 @@
           "status": {
             "$ref": "#/components/schemas/VenvProbeStatus"
           },
+          "uv_available": {
+            "type": "boolean",
+            "description": "本机 PATH 中是否能找到 uv"
+          },
           "venv_path": {
             "type": [
               "string",