container: generalise background from Color to Paint

`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`.

src/draw/layout.rs

@@ -164,9 +164,9 @@ pub( crate ) fn layout_and_draw<Msg: Clone>(
 			};
 			if !painted
 			{
-				if let Some( bg ) = c.background
+				if let Some( ref bg ) = c.background
 				{
-					canvas.fill_rect( rect, bg, c.corners );
+					canvas.fill_paint_rect( rect, bg, c.corners );
 				}
 			}
 			if let Some( ( color, width ) ) = c.border

src/widget/container.rs

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: LGPL-2.1-only
 // Copyright (C) 2026 Liberux Labs, S. L. <info@liberux.net>
 
+use crate::theme::Paint;
 use crate::types::{ Color, Corners };
 use crate::render::Canvas;
 use super::Element;
@@ -50,7 +51,11 @@ use super::Element;
 pub struct Container<Msg: Clone>
 {
 	pub child:      Box<Element<Msg>>,
-	pub background: Option<Color>,
+	/// Optional background paint — flat colour, linear or radial
+	/// gradient. Constructed via [`Container::background`], which
+	/// accepts anything `Into<Paint>` (a plain [`Color`] gets
+	/// promoted to [`Paint::Solid`] via the trait impl).
+	pub background: Option<Paint>,
 	/// Slot id of a themed surface (resolved via
 	/// [`crate::theme::resolve_surface`]). When set, takes precedence
 	/// over `background` and paints the full Glass stack instead of a
@@ -109,12 +114,15 @@ impl<Msg: Clone> Container<Msg>
 		self
 	}
 
-	/// Set the background fill color. Ignored at paint time if a
+	/// Set the background fill. Accepts anything convertible to
+	/// [`Paint`] — a plain [`Color`] (auto-wrapped in
+	/// [`Paint::Solid`]) or an explicit [`crate::theme::LinearGradient`]
+	/// / [`crate::theme::RadialGradient`]. Ignored at paint time if a
 	/// themed [`surface`](Self::surface) is set and resolves against
 	/// the active theme.
-	pub fn background( mut self, color: Color ) -> Self
+	pub fn background( mut self, paint: impl Into<Paint> ) -> Self
 	{
-		self.background = Some( color );
+		self.background = Some( paint.into() );
 		self
 	}