sar3d: fixed default scene window X [-2,2], Y [-0.5,2], Z [0,8] m

The grid bounds are now explicit config (sar3d_x/y/z_min/max_m) instead of
being derived from the antenna span and the depth gate. The depth mask
still zeroes voxels outside [min_depth, max_depth], so the wider Z window
just shows the masked wedge. The bound fields are double: the parity tool
recomputes the notebook-derived bounds (notebook_grid_bounds) and passes
them through the live config, and a float roundtrip shifted the axes by
~1e-7 m, flipping depth-mask knife-edge voxels against the reference npz.
Parity on frames 0034-0036 unchanged (1.3e-6, axes bit-match).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

python_app/models/gui_profile_schema.py

@@ -148,9 +148,12 @@ class GuiSar3dStateModel:
     pitch_override_enabled: bool = False
     pitch_deg: float = 17.5
     ground_depth_m: float = 1.25
-    x_margin_m: float = 2.0
+    x_min_m: float = -2.0
+    x_max_m: float = 2.0
     y_min_m: float = -0.5
     y_max_m: float = 2.0
+    z_min_m: float = 0.0
+    z_max_m: float = 8.0
     voxel_x_m: float = 0.025
     voxel_y_m: float = 0.02
     voxel_z_m: float = 0.02