fix: FadingClip gradient colour mismatch on hover/press

- composite() now blends in linear RGB to match iced's GPU pipeline
  (sRGB framebuffer + PREMULTIPLIED_ALPHA_BLENDING), fixing the visible
  seam between the gradient and button background in popup list items.

- Add composite_srgb() for the panel bar, which is rendered by the
  Wayland compositor and may blend in sRGB rather than linear.

- list_item() uses Button::Custom that delegates to MenuItem for every
  state except pressed (reuses hovered style), so the FadingClip
  gradient matches the button background in all interactive states
  without fragile pressed-state tracking.

- Remove mouse_pressed tracking from FadingClipState — no longer needed
  now that the button class guarantees hover == pressed colours.

Author: glima