Fix tmux and nvim color display issues

- Add COLORTERM=truecolor to zsh session variables for 24-bit color support
- Update tmux terminal overrides to match xterm-256color terminal type
- Add gruvbox-nvim plugin and set explicit background=dark in nvim config
- Replace default colorscheme with gruvbox to match lualine theme

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

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

tmux.nix

@@ -12,7 +12,7 @@
     extraConfig = ''
       # Terminal settings
       set -g default-terminal "screen-256color"
-      set -ga terminal-overrides ",*256col*:Tc"
+      set -ga terminal-overrides ",xterm-256color:Tc"
       
       # Gruvbox Dark Theme
       # Status bar colors