Add GTK wallpaper library

Scan ~/Pictures/Wallpapers and /usr/share/backgrounds/bos (configurable)
and open a bread-theme GTK picker via `breadpaper library` (alias browse).
Clicking a thumbnail runs the existing set path. listen honors
bread.command.paper.library by spawning that picker.

.forgejo/workflows/rc-release.yml

@@ -37,6 +37,7 @@ jobs:
           sha256sum "${PKG_DIR}/breadpaper-x86_64" | awk '{print $1}' \
             > "${PKG_DIR}/breadpaper-x86_64.sha256"
           cp src/LICENSE "${PKG_DIR}/"
+          cp src/packaging/breadpaper.desktop "${PKG_DIR}/"
           cp src/bakery.toml "${PKG_DIR}/bakery.toml"
           ln -sfn "${VERSION}" "/srv/breadway-dl/beta/breadpaper/latest"