Fix wallpaper upgrade compatibility and Settings icon clipping

Allow installed releases to omit the two newer appearance wallpapers while keeping new bundle verification strict. Fit the Settings header symbol within its square slot without clipping.

Verified 30 installer tests, the real upgrade dry-run, and release rendering previews on GPU and software backends.

shell/ferese-settings/src/visuals.rs

@@ -34,6 +34,7 @@ pub fn brand_icon() -> svg_icon::Icon {
         .symbolic(false)
         .icon()
         .size(32)
+        .content_fit(cosmic::iced::ContentFit::Contain)
 }
 
 pub fn action_icon(path: &str, tint: Color) -> svg_icon::Icon {