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>

flake.nix

@@ -52,6 +52,9 @@
         enable = true;
         defaultKeymap = "viins";
         enableVteIntegration = true;
+        sessionVariables = {
+          COLORTERM = "truecolor";
+        };
       };
     };
   in