feat: editable grid spacing and major grid lines in Drafting Settings

The Snap and Grid tab showed static grid values with dead checkboxes, and every grid line stroked identically.

Grid X/Y spacing and Major-line-every are now editable inputs (validated 2-100 for majors, dirty-tracked, applied with an error message on bad input); Adaptive-grid and Display-beyond-Limits checkboxes are wired to live state. Every Nth line partitions into major_segments and strokes subtly brighter (1.5x alpha, 0.75px) for the reference hierarchy. GRIDMAJOR state persists to UserSettings. Covers partition, invalidation, and parse tests.

src/app/view/mod.rs

@@ -465,6 +465,7 @@ impl OpenCADStudio {
                         bounds,
                         step_x,
                         step_y,
+                        major_every: self.grid_major_every,
                         origin,
                         axes,
                         limits,