Replace text-based prefix indicator with session name color change

- Remove asymmetric <Prefix> text indicator from status-right
- Change session name background color when prefix is active:
  - Normal: gray background (#a89984)
  - Prefix active: yellow background (#fabd2f)
- Simplify time display to consistent ' %H:%M ' format
- Provides cleaner, more elegant visual prefix indication

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

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

modules/tmux.nix

@@ -36,8 +36,8 @@
       # Status bar content
       set -g status-left-length 40
       set -g status-right-length 30
-      set -g status-left ' #S '
+      set -g status-left '#{?client_prefix,#[bg=#fabd2f],#[bg=#a89984]}#[fg=#282828] #S #[default]'
-      set -g status-right '#{?client_prefix,#[reverse]<Prefix>#[noreverse] ,} %H:%M '
+      set -g status-right ' %H:%M '
       
       # Window status format
       set -g window-status-format ' #I:#W '