feat(background.py): 启用背景校正图表生成

algorithmic_baseline() 函数：
- 原代码: fig = None  # plotting disabled
- 修复: 调用 plotting.background_plotting(df, gas) 生成交互式图表，
  显示原始浓度、拟合基线、信号点（红色）和背景点（蓝色圆圈）

异常保护：try/except 包裹，绘图失败时回退至 None

src/gasflux/background.py

@@ -52,7 +52,11 @@ def algorithmic_baseline(
     signal = (df[gas] - bkg)[~bkg_points]
     df[f"{gas}_signal"] = np.invert(bkg_points)
 
-    fig = None  # plotting disabled
+    try:
+        from . import plotting
+        fig = plotting.background_plotting(df, gas)
+    except Exception:
+        fig = None
     output_text = (
         f"Baseline algorithm: {algorithm}\n"
         f"Positive and negative 95% percentile of baseline: {np.percentile(background, 2.5):.2f} ppm, "