Use gruvbox purple for prefix indicator to improve visibility

Change prefix session name background from yellow (#fabd2f) to purple
(#b16286) to avoid confusion with active window color. Purple provides
better visual distinction and is not used elsewhere in the tmux theme.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

modules/tmux.nix

@@ -36,7 +36,7 @@
       # Status bar content
       set -g status-left-length 40
       set -g status-right-length 30
-      set -g status-left '#{?client_prefix,#[bg=#fabd2f],#[bg=#a89984]}#[fg=#282828] #S #[default]'
+      set -g status-left '#{?client_prefix,#[bg=#b16286],#[bg=#a89984]}#[fg=#282828] #S #[default]'
       set -g status-right ' %H:%M '
       
       # Window status format