feat: Grid snap adaptive spacing + Polar tracking improvements

Grid snap (F9):
  • snap spacing now tracks the visible grid zoom level — computed via
    compute_grid_step() (same adaptive pow-of-5 logic as draw_grid),
    updated on every ViewportMove so zooming in/out keeps snap aligned
    with the grid lines on screen

Polar tracking (F10):
  • configurable angle increment stored in polar_increment_deg (default 45°)
  • right-click the POLAR pill to cycle through 15 → 30 → 45 → 90°
  • pill label shows current angle ("POLAR 45°")
  • visual guide line: while drawing with polar on, a dashed cyan line is
    drawn from last_point along the snapped direction across the viewport
  • SetPolarAngle message + handler; all polar_constrain() calls use the
    stored increment instead of the hardcoded 45°

ROADMAP updated: Grid snap and Polar tracking marked ✅.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/view.rs

@@ -177,6 +177,7 @@ impl H7CAD {
                     self.snap_popup_open,
                     self.ortho_mode,
                     self.polar_mode,
+                    self.polar_increment_deg,
                     self.show_grid,
                     tab.scene.layout_names(),
                     tab.scene.current_layout.clone(),