fix: 折线图纵轴优化——默认约80%头空间、取整到nice刻度，支持手动设置纵轴上限

- 纵轴自动刻度改为精细步进(1/1.5/2/2.5/3/4/5/6/8/10)，数据最大值落在约80%处
- ChartInput 新增 yMax 字段(JSON 存储、可拓展)，图表弹窗新增「纵轴上限」输入
- 新增 yMax 往返测试

tests/test_charts.py

@@ -29,6 +29,14 @@ def test_chart_crud_and_conflict(tmp_path):
     assert repo.delete_chart(chart["chartId"])["deleted"] is True
 
 
+def test_chart_yMax_roundtrip(tmp_path):
+    repo = JsonRepository(tmp_path)
+    bounded = repo.create_chart(ChartInput(title="带上限", yMax=10))
+    assert repo.get_chart(bounded["chartId"])["yMax"] == 10
+    auto = repo.create_chart(ChartInput(title="自动"))
+    assert repo.get_chart(auto["chartId"])["yMax"] is None
+
+
 def test_chart_data_modes_and_continuous_points(tmp_path):
     repo = JsonRepository(tmp_path)
     task = _task(repo)