feat: 添加 nvim-tree 侧边文件树

- 新增 filetree.lua：nvim-tree 常驻侧边栏
- <leader>e 开关侧边文件树（VSCode 风格）
- oil.nvim 保留，用 - 键快速编辑目录
- bufferline 添加 NvimTree 偏移适配
- which-key 添加 <leader>e 分组

lua/plugins/filetree.lua

@@ -0,0 +1,68 @@
+-- =============================================================================
+-- filetree.lua — 侧边文件树
+-- =============================================================================
+-- 【新增文件】nvim-tree 作为常驻侧边栏（VSCode 风格），oil.nvim 保留用于快速编辑。
+-- 两者共存：nvim-tree 看结构，oil 改文件。
+-- 键位：<leader>e 开关侧边栏
+
+return {
+  {
+    "nvim-tree/nvim-tree.lua",
+    version = "*",
+    lazy = false,  -- 立即加载，确保打开就有侧边栏
+    dependencies = { "nvim-tree/nvim-web-devicons" },
+    config = function()
+      -- 禁用 netrw（避免和 oil 冲突，oil 自己也会禁一次，这里双保险）
+      vim.g.loaded_netrw = 1
+      vim.g.loaded_netrwPlugin = 1
+
+      require("nvim-tree").setup({
+        sort_by = "case_sensitive",
+        view = {
+          width = 30,
+          side = "left",
+          relativenumber = false,
+          -- ✅ 关闭文件树时自动调整窗口大小
+          adaptive_size = false,
+        },
+        renderer = {
+          group_empty = true,
+          icons = {
+            show = {
+              file = true,
+              folder = true,
+              folder_arrow = true,
+              git = true,
+            },
+          },
+        },
+        filters = {
+          dotfiles = false,
+          custom = { "^.git$" },
+        },
+        git = {
+          enable = true,
+          ignore = false,
+        },
+        actions = {
+          open_file = {
+            quit_on_open = false,  -- 打开文件不关树
+            window_picker = {
+              enable = true,
+            },
+          },
+        },
+        -- ✅ 打开 nvim 时自动打开文件树
+        auto_open_on_setup = false,  -- 不自动开，用 <leader>e 手动开
+        on_attach = function(bufnr)
+          local api = require("nvim-tree.api")
+          api.config.mappings.default_on_attach(bufnr)
+        end,
+      })
+
+      -- <leader>e 开关侧边文件树
+      vim.keymap.set("n", "<leader>e", "<cmd>NvimTreeToggle<CR>",
+        { desc = "侧边文件树", noremap = true, silent = true })
+    end,
+  },
+}

lua/plugins/ui.lua

@@ -43,7 +43,7 @@ return {
           tab_size = 18,
           diagnostics = "nvim_lsp",
           diagnostics_update_in_insert = false,
-          -- ✅ 适配 oil.nvim：左侧文件浏览器偏移
+          -- ✅ 适配 oil.nvim 和 nvim-tree：左侧文件浏览器偏移
           offsets = {
             {
               filetype = "oil",
@@ -51,6 +51,12 @@ return {
               highlight = "Directory",
               text_align = "left",
             },
+            {
+              filetype = "NvimTree",
+              text = "文件树",
+              highlight = "Directory",
+              text_align = "left",
+            },
           },
           color_icons = true,
           show_buffer_icons = true,

lua/plugins/whichkey.lua

@@ -41,6 +41,7 @@ return {
 
       -- ✅ 新增：键位分组提示
       wk.add({
+        { "<leader>e", group = "文件树" },
         { "<leader>f", group = "搜索" },
         { "<leader>g", group = "Git" },
         { "<leader>h", group = "Hunk 操作" },