Settings: Show grid toggle + grid aligned to active sketch plane

[trailcode]

Closes #128
Closes #130

Summary

Two related 3D view grid improvements:

1. Show grid checkbox in Settings → 3D view grid (persisted as occt_view.grid_visible)
2. Grid aligned to active sketch plane via V3d_Viewer::SetPrivilegedPlane from the current sketch

Changes

Show / hide grid (#128)

• Occt_view::get_grid_visible() / set_grid_visible() / apply_grid_visibility_()
• Settings UI + JSON load/save
• Default grid_visible: true in res/ezycad_settings.json

Active sketch plane (#130)

• sync_grid_plane_to_active_sketch_() / refresh_viewer_grid_()
• Refresh grid when switching sketches, loading documents, adding sketches, and editing grid settings
• Initial grid display deferred until after init_default (first sketch exists)

Docs

• docs/usage-settings.md — Show grid control, grid_visible JSON key, note that grid follows active sketch

Test plan

• Settings → 3D view grid (verbosity ≥ 3): toggle Show grid — grid hides/shows immediately
• Switch between sketches with different planes — grid moves to active sketch plane
• Open a multi-sketch .ezy — grid matches current sketch after load
• Restart app — grid_visible preference restored
• Desktop Release build (EzyCad_lib)

Commits

• 26069ef — Show grid checkbox
• c128baa — Grid synced to active sketch plane