refactor: 重构为多平台多用户架构，全面提升终端体验

- 新增 lib/ 辅助函数（mkNixos/mkDarwin/mkHome），消除硬编码
- 拆分 modules/ 为 nixos/darwin/shared 三层，支持跨平台共享
- 重构 home/ 为 shell/dev/theme 子模块，工具全部迁入用户级
- 深度配置 Tmux/FZF/Atuin/Bat/Eza/Btop/Lazygit/Starship
- 添加 Catppuccin Mocha 统一主题、Nerd Fonts 声明式管理
- 新增 homeConfigurations 输出，支持非 NixOS 系统直装
- 新增 nix-darwin 输入，预留 macOS 扩展路径
- 新增 overlays/ 和 pkgs/ 自定义包基础设施
- 修正 nix-darwin URL（LnL7 → nix-darwin org）

home/shell/tmux.nix

@@ -0,0 +1,51 @@
+{ pkgs, ... }:
+
+{
+  programs.tmux = {
+    enable = true;
+    baseIndex = 1;
+    escapeTime = 10;
+    historyLimit = 50000;
+    keyMode = "vi";
+    mouse = true;
+    terminal = "tmux-256color";
+    prefix = "C-a";
+
+    plugins = with pkgs.tmuxPlugins; [
+      sensible
+      vim-tmux-navigator
+      yank
+    ];
+
+    extraConfig = ''
+      # ── True color support ──
+      set -ag terminal-overrides ",xterm-256color:RGB"
+
+      # ── Split panes ──
+      bind v split-window -h -c "#{pane_current_path}"
+      bind s split-window -v -c "#{pane_current_path}"
+      unbind '"'
+      unbind %
+
+      # ── New window in current path ──
+      bind c new-window -c "#{pane_current_path}"
+
+      # ── Reload config ──
+      bind r source-file ~/.config/tmux/tmux.conf \; display "Config reloaded!"
+
+      # ── Resize panes ──
+      bind -r H resize-pane -L 5
+      bind -r J resize-pane -D 5
+      bind -r K resize-pane -U 5
+      bind -r L resize-pane -R 5
+
+      # ── Status bar ──
+      set -g status-position top
+      set -g renumber-windows on
+
+      # ── Copy mode vim bindings ──
+      bind -T copy-mode-vi v send-keys -X begin-selection
+      bind -T copy-mode-vi y send-keys -X copy-selection-and-cancel
+    '';
+  };
+}