fix(hatch): radial gradients and correct gradient stop colours

A gradient hatch was drawn linear from the entity colour to its second
stop, so a red->yellow spherical fill came out as a pale linear wash. Two
fixes:
- Colour: start from the gradient's first stop, not the entity colour.
  hatch_model_from_dxf already baked stop-0 in; stop synced_hatch_models
  from clobbering it with the entity's resolved colour for gradient fills.
- Shape: spherical / hemispherical / curved gradient names now render as a
  radial fill (new shader mode 3, centred on the boundary with radius to the
  farthest vertex) instead of linear. Native and web shaders both updated.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/modules/draw/draw/hatch.rs

@@ -395,6 +395,7 @@ impl GradientCommand {
             pattern: HatchPattern::Gradient {
                 angle_deg: 0.0,
                 color2: [0.18, 0.18, 0.18, 0.0],
+                radial: false,
             },
             name: "LINEAR".into(),
             color: [0.30, 0.60, 0.95, 0.80],