`Container::background` now stores `Option<Paint>` instead of `Option<Color>`, and the builder accepts `impl Into<Paint>` so callers can pass a plain `Color` (auto-wrapped in `Paint::Solid` via the trait impl) or an explicit `LinearGradient` / `RadialGradient`. `layout_and_draw` switches from `canvas.fill_rect( rect, bg, corners )` to `canvas.fill_paint_rect( rect, &bg, corners )` to consume the wider type.
No behaviour change for existing call sites — solid-colour containers keep working unchanged thanks to the `Into<Paint>` for `Color`.
