收紧绘图设置为两页，并把 FN 牛顿下限固定为 0。

手指 2D 方向改到 ArrowPanel 设置对话框；归零阈值不变。已用 display_prefs、finger_2d_orientation 与绘图对话框相关 smoke 测试验证。

apps/studio/tests/test_display_prefs.py

@@ -257,12 +257,12 @@ def test_applying_plot_settings_writes_the_preferences_and_the_panel():
         }
     )
     assert parts["host"].prefs["plot_window_sec"] == 20.0
-    assert parts["host"].prefs["plot_fn_min"] == -2.0
+    assert parts["host"].prefs["plot_fn_min"] == 0.0
     assert parts["host"].prefs["plot_shear_raw_max"] == 3e5
     assert parts["host"].prefs["plot_deadband_n"] == 0.25
     assert parts["charts"].window_sec == [20.0]
     assert parts["charts"].limits[-1] == {
-        "fn_min": -2.0,
+        "fn_min": 0.0,
         "fn_max": 8.0,
         "shear_max": 9.0,
         "fn_raw_min": -1000.0,
@@ -281,8 +281,9 @@ def test_a_partial_plot_settings_mapping_leaves_the_rest_as_stored():
         {"window_sec": 6.0, "fn_max": 12.0, "shear_max": 3.0,
          "fn_raw_max": 4e5, "shear_raw_max": 5e5}
     )
-    assert parts["host"].prefs["plot_fn_min"] == -5.0
-    assert parts["charts"].limits[-1]["fn_min"] == -5.0
+    # Newton FN floor is always rewritten to 0; other stored values stay.
+    assert parts["host"].prefs["plot_fn_min"] == 0.0
+    assert parts["charts"].limits[-1]["fn_min"] == 0.0
     assert parts["charts"].deadband_floors[-1]["raw"] == 900.0