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.

kde/fastfetch/README.md

@@ -0,0 +1,26 @@
+# Fastfetch for Konsole
+
+From the repository root:
+
+```sh
+stow -d kde -t "$HOME" fastfetch
+```
+
+The configuration, source PNG, and generated logo live in
+`kde/fastfetch/.config/fastfetch/`. The generated logo is included, so a new
+machine does not need to run the build script.
+
+Choose either this package or `mac/fastfetch`; both own `~/.config/fastfetch/`.
+
+Konsole needs inline image data instead of the original `kitty-direct` file
+transfer. The committed `albedo-konsole.kitty` logo is pre-scaled to 20 × 10
+cells to avoid grainy terminal scaling.
+
+After changing `albedo.png`, the logo dimensions, or Konsole's font, update
+the cell size in `build.py` and regenerate the logo from the repository root:
+
+```sh
+python3 kde/fastfetch/build.py
+```
+
+The build script requires `vipsthumbnail` from libvips.