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};

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

@@ -0,0 +1,62 @@
+use gpui::{
+    Hsla, IntoElement, ParentElement, Styled, div, hsla, linear_color_stop, linear_gradient, rgb,
+};
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub(crate) enum WallpaperTheme {
+    Dawn,
+}
+
+impl WallpaperTheme {
+    fn base(self) -> u32 {
+        match self {
+            Self::Dawn => 0xefe8e1,
+        }
+    }
+
+    fn upper_blob(self) -> Hsla {
+        match self {
+            Self::Dawn => hsla(351.0 / 360.0, 1.0, 0.91, 0.85),
+        }
+    }
+
+    fn lower_blob(self) -> Hsla {
+        match self {
+            Self::Dawn => hsla(228.0 / 360.0, 1.0, 0.86, 0.65),
+        }
+    }
+}
+
+pub(crate) fn render_wallpaper(theme: WallpaperTheme) -> impl IntoElement {
+    let upper = theme.upper_blob();
+    let lower = theme.lower_blob();
+
+    div()
+        .absolute()
+        .inset_0()
+        .bg(rgb(theme.base()))
+        .child(
+            div()
+                .absolute()
+                .inset_0()
+                .bg(linear_gradient(
+                    225.0,
+                    linear_color_stop(upper, 0.0),
+                    linear_color_stop(transparent_like(upper), 0.55),
+                )),
+        )
+        .child(
+            div()
+                .absolute()
+                .inset_0()
+                .bg(linear_gradient(
+                    45.0,
+                    linear_color_stop(lower, 0.0),
+                    linear_color_stop(transparent_like(lower), 0.62),
+                )),
+        )
+}
+
+fn transparent_like(color: Hsla) -> Hsla {
+    hsla(color.h, color.s, color.l, 0.0)
+}