refactor: reorganize flake structure and consolidate user config

Directory structure:
- Move from src/ to root level (system/, user/)
- Remove unused machines (workstation, vm, laptop)

User configuration:
- Add user/home.nix for shared defaults (pass, essentials, default modules)
- Centralize user options in user/default.nix
- Move submodules to consistent paths (bash/bash, git/git, neovim/nvim, vim/vim)

Module reorganization:
- Flatten nested module structures (remove /modules/ subdirs)
- Split CLI vs GUI tools (dev/ for CLI, gui/dev/ for GUI)
- Move neovim/vim to top-level modules (not under utils/)
- Remove security.enable - pass now in user/home.nix
- Remove utils.enable - essentials now in user/home.nix
- Add security/yubikey module with yubikey-manager, age-plugin-yubikey
- Move pcb, design to gui/dev/
- Replace penpot docker wrapper with nixpkgs penpot-desktop
- Remove i3 config
- Remove deprecated wsl.nativeSystemd option

GUI improvements:
- Browser-focused mimeApps in gui/default.nix
- Each WM handles its own auto-start via profileExtra

Cleanup:
- Update README with new structure
- Update justfile paths and valid systems
- Fix submodule paths in .gitmodules

user/modules/tmux/default.nix

@@ -0,0 +1,44 @@
+{ pkgs, lib, config, ... }:
+
+with lib;
+let
+  cfg = config.modules.user.tmux;
+  gui = config.modules.user.gui.wm;
+
+  wm = {
+    enable = builtins.any (mod: mod.enable or false) (builtins.attrValues gui);
+  };
+
+in
+{ options.modules.user.tmux = { enable = mkEnableOption "Enable tmux module"; };
+  config = mkIf cfg.enable {
+    programs.tmux = {
+      enable = true;
+      newSession = true;
+      disableConfirmationPrompt = true;
+      keyMode = "vi";
+      mouse = if wm.enable then true else false;
+
+      prefix = "M";
+      #shell = "\${pkgs.bash}/bin/bash";
+
+      plugins = with pkgs.tmuxPlugins; [
+        {
+          plugin = tilish;
+          extraConfig = ''
+            set -g @tilish-default 'tiled'
+          '';
+        }
+      ];
+
+      extraConfig = import ./config/tmux.nix;
+    };
+
+    # Auto-start tmux only on local TTY (not SSH, not in tmux already)
+    programs.bash.profileExtra = mkIf (!wm.enable) ''
+      if [[ $- == *i* ]] && [ -z "$DISPLAY" ] && [ -z "$TMUX" ] && [ -z "$SSH_TTY" ] && [ -z "$SSH_CONNECTION" ]; then
+        exec tmux
+      fi
+    '';
+  };
+}