Fix gap color between session name and window tags

- Explicitly set gap background to main status bar color (#282828)
- Prevents gap from inheriting session section's background color
- Now properly matches the main status bar background

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

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

modules/tmux.nix

@@ -41,7 +41,7 @@
       # Status bar content
       set -g status-left-length 40
       set -g status-right-length 30
-      set -g status-left '#{?client_prefix,#[bg=#fb4934],#[bg=#a89984]}#[fg=#282828] #S #[default] '
+      set -g status-left '#{?client_prefix,#[bg=#fb4934],#[bg=#a89984]}#[fg=#282828] #S #[bg=#282828] '
       set -g status-right '#{?pane_in_mode,[COPY],} %H:%M '
       
       # Window status format