fix: set the role colour after .p10k.zsh, and add the config alias

The colour was in 05-prompt.zsh, before oh-my-zsh. That is right for ZSH_THEME
and wrong for everything else: ~/.p10k.zsh assigns
POWERLEVEL9K_CONTEXT_BACKGROUND itself and is sourced from 30-path.zsh, so the
role colour was overwritten before the first prompt was drawn. The isolated
test missed it because the throwaway HOME had no .p10k.zsh; on the real machine
the desktop came up with 0 instead of blue.

Theme before oh-my-zsh, colour after .p10k.zsh - two different constraints that
happen to pull in opposite directions.

The config alias was missing entirely. Without it the bare repository has no
front end, which makes the whole arrangement unusable. Guarded on ~/.cfg
existing so the alias does not appear on a machine that has not been set up.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.config/zsh/rc.d/50-desktop.zsh

@@ -38,3 +38,9 @@ for _d in "$HOME/.cargo/bin" "$HOME/.lmstudio/bin"; do
 done
 unset _d
 export PATH
+
+# Role colour, set here rather than in 05-prompt.zsh because ~/.p10k.zsh
+# assigns POWERLEVEL9K_CONTEXT_BACKGROUND itself and is sourced in 30-path.zsh.
+# Anything set before that is silently replaced. Not decoration: shron carries
+# mail, web and cloud, and a glance should say which machine this is.
+typeset -g POWERLEVEL9K_CONTEXT_BACKGROUND=blue