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/README.md

@@ -0,0 +1,37 @@
+# KDE Plasma packages
+
+From the repository root, install all KDE packages:
+
+```sh
+(cd kde && stow -t "$HOME" */)
+```
+
+Or choose individual packages:
+
+```sh
+stow -d kde -t "$HOME" spectacle fastfetch
+```
+
+| Package                            | Details                                                  |
+| ---------------------------------- | -------------------------------------------------------- |
+| [theme](theme/README.md)           | Darwin Breeze KDE colors and generated app themes        |
+| [helium](helium/README.md)         | Browser flags and the generated Helium theme             |
+| [fastfetch](fastfetch/README.md)   | Konsole configuration and a pre-scaled inline image logo |
+| [spectacle](spectacle/README.md)   | Screenshots to clipboard with a notification, no preview |
+| [tray-icons](tray-icons/README.md) | Ngó Sen and qBittorrent tray icons                       |
+
+Theme and Spectacle need an additional apply step after stowing their package:
+
+```sh
+# If you selected theme:
+./kde/theme/apply.sh
+# If you selected spectacle:
+./kde/spectacle/apply.py
+```
+
+The apply scripts require a running KDE Plasma 6 session. Ngó Sen icons need
+an additional install step described in the tray-icons README.
+
+Packages can be selected independently. Helium includes its generated theme;
+it does not require the KDE theme to be installed. Choose either
+`kde/fastfetch` or `mac/fastfetch`; both own `~/.config/fastfetch/`.