Decode webp in the shell (#7477)

The background and the lock screen are drawn by Quickshell, so they decode
through Qt, which ships handlers for png, jpeg and gif but not webp. Any
third-party theme shipping a .webp background comes up blank without the
qt6-imageformats plugin, and extra themes are their own repositories.

Only the dependency is taken here. The background conversion that came with
it re-encodes all 79 bundled backgrounds, which 4.0.1 has no need of.

(cherry picked from commit a4219f8fbeb85ee7a2bc0e14eda9d38cc85d3d0d, dependency only)

install/omarchy-base.packages

@@ -108,6 +108,7 @@ python-poetry-core
 ttfx
 qemu-user-static-binfmt
 qrencode
+qt6-imageformats
 quickshell-git
 ripgrep
 ruby

migrations/1787133200.sh

@@ -0,0 +1,3 @@
+echo "Add webp decoding to the shell"
+
+omarchy-pkg-add qt6-imageformats