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.

common/README.md

@@ -0,0 +1,18 @@
+# Common packages
+
+Shared configuration for Git, Micro, mpv, npm, Starship, and Zsh.
+From the repository root, install all shared packages:
+
+```sh
+(cd common && stow -t "$HOME" */)
+```
+
+Or choose individual packages:
+
+```sh
+stow -d common -t "$HOME" git
+stow -d common -t "$HOME" zsh starship
+```
+
+Choose any of `git`, `micro`, `mpv`, `npm`, `starship`, and `zsh`.
+Reload Zsh with `exec zsh` after installing its configuration.