refactor: group stow packages into common, mac and kde

Split the packages by platform so each app stays an opt-in Stow package
under common/, mac/ or kde/. KDE gets its own Fastfetch, Helium and
Spectacle packages, and darwin-breeze becomes kde/theme. The READMEs
document stowing a whole group as well as individual packages.

mac/README.md

@@ -0,0 +1,21 @@
+# macOS packages
+
+From the repository root, install all macOS packages:
+
+```sh
+(cd mac && stow -t "$HOME" */)
+```
+
+Or choose individual packages:
+
+```sh
+stow -d mac -t "$HOME" ghostty
+stow -d mac -t "$HOME" fastfetch
+```
+
+- `ghostty`: JetBrainsMono Nerd Font and Ghostty's bundled SpaceGray Eighties theme.
+- `fastfetch`: the original configuration, with a `kitty-direct` image logo.
+  Its source PNG is included in `fastfetch/.config/fastfetch/`.
+
+These packages are independent. Choose either `mac/fastfetch` or
+`kde/fastfetch`; both own `~/.config/fastfetch/`.