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,
+  },
+}