Editor: solid→solid paint conversion squares the alpha

[softmarshmallow]

Found by review during #94, but pre-existing — the behaviour is unchanged by that PR, so it was filed rather than folded into a rendering rung.

What

convert_paint in crates/grida_editor/src/ui/bind.rs folds the source paint's opacity into the first stop's alpha when converting to a solid (kind 0).

For a source that is already Paint::Solid, both inputs are the same number:

• paint_stops(paint) for Paint::Solid returns a first stop whose colour is p.color, alpha included.
• SolidPaint::opacity() returns self.color.a() as f32 / 255.0.

So first_color.a() * opacity squares the alpha. A 50%-opaque solid converted to a solid becomes ~25% opaque.

FillKind and StrokeKind can both reach convert_paint with index 0 while the paint is already solid.

Suggested shape

Return paint.clone() when the source is already Paint::Solid and the target kind is solid, before the alpha fold; keep the existing fold for gradient and image sources, where the two numbers are genuinely independent.

Needs a test in crates/grida_editor/tests/paint_contracts.rs alongside the existing fill_kind_converts_carrying_the_color, which only covers an opaque colour and so cannot see this.