fix(viewport): stabilize grid and grip clipping

Keep the grid on UCS XY, derive spacing from zoom, and clip perspective lines by screen density without dropping the near side.

src/app/helpers.rs

@@ -1,4 +1,3 @@
-use crate::ui::overlay::GridPlane;
 use acadrust::tables::Ucs;
 
 // ── Coordinate parsing ─────────────────────────────────────────────────────
@@ -160,22 +159,6 @@ pub(super) fn ucs_rotated_z(origin: glam::DVec3, angle_z: f32) -> Ucs {
     ucs
 }
 
-// ── Grid plane detection ───────────────────────────────────────────────────
-
-/// Choose the grid plane whose normal is most aligned with the camera view direction.
-pub(super) fn grid_plane_from_camera(pitch: f32, yaw: f32) -> GridPlane {
-    let fz = pitch.sin().abs();
-    let fy = (pitch.cos() * yaw.cos()).abs();
-    let fx = (pitch.cos() * yaw.sin()).abs();
-    if fz >= fy && fz >= fx {
-        GridPlane::Xy
-    } else if fy >= fx {
-        GridPlane::Xz
-    } else {
-        GridPlane::Yz
-    }
-}
-
 // ── Drawing constraint helpers ─────────────────────────────────────────────
 
 /// Constrain `pt` to the nearest 90° direction from `base`, in the active UCS