Match shell squircle outlines with compositor masks

Use shared corner geometry for shell controls, window clipping, materials and blur masks. Preserve circular pills and controls where appropriate, track contour changes in damage, and use tagged ferese-shape and libcosmic dependencies.

crates/ferese-theme/src/menus.rs

@@ -1,5 +1,6 @@
 //! Shared menu layouts and paint; callers own actions and animation timing.
 use cosmic::Element;
+use cosmic::iced::border::Shape as BorderShape;
 use cosmic::iced::{Alignment, Color, Length};
 use cosmic::widget::{button, container, row as iced_row};
 
@@ -115,11 +116,13 @@ pub fn slider(foreground: Color, opacity: f32) -> cosmic::theme::iced::Slider {
                 ),
                 width: 4.,
                 border: cosmic::iced::Border {
+                    shape: BorderShape::Continuous,
                     radius: 2.into(),
                     ..Default::default()
                 },
             },
             handle: Handle {
+                corner_shape: BorderShape::Circular,
                 shape: HandleShape::Circle { radius: 6. },
                 background: foreground.into(),
                 border_width: 0.,