Add layered dawn wallpaper to shell background

Designs call for a radial-gradient dawn (cream base + pink upper-right
glow + blue lower-left glow). GPUI 0.2.2 only supports linear gradients,
so the wallpaper is composed of a cream base layer with two diagonal
linear gradients that fade to transparent at ~60% — the perceived effect
matches the design's softness without needing radial-gradient support.

Wallpaper is isolated to chrome::wallpaper so future appearance themes
(violet, mint, slate) extend the enum without touching the shell.

crates/ely_app/src/shell/chrome/mod.rs

@@ -0,0 +1,3 @@
+pub(crate) mod wallpaper;
+
+pub(crate) use wallpaper::{WallpaperTheme, render_wallpaper};