Route domain WallpaperTheme through render_wallpaper

The shell now reads snapshot.appearance.wallpaper and feeds the domain
WallpaperTheme directly into render_wallpaper. The chrome enum is
deleted; the four design themes (Dawn / Violet / Mint / Slate) all map
to base + upper-blob + lower-blob HSLA palettes that approximate the
design's radial gradients with GPUI's 2-stop linear gradient pair.

Switching themes via core.set_wallpaper_theme now propagates through
snapshots into the next render.

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

@@ -20,4 +20,4 @@ pub(crate) use split_pane::{
     render_split_pane_header,
 };
 pub(crate) use topbar::render_topbar;
-pub(crate) use wallpaper::{WallpaperTheme, render_wallpaper};
+pub(crate) use wallpaper::render_wallpaper;