feat: 重新设计现代化图标系统

新增 Icons.tsx 统一图标组件（25 个图标）：
- AppIcon: 渐变蓝色圆角矩形 + M↓ 符号
- 工具栏: FolderOpen, Save, SplitView, EditMode, PreviewMode, Moon, Sun
- 标签栏: Close, Plus
- 侧边栏: Folder, File, ChevronRight, FolderPlus
- 搜索栏: ChevronUp, ChevronDown, X
- 拖拽: UploadCloud
- 导航: ArrowUp, ArrowDown
- 欢迎: WelcomeFile, WelcomeNew

设计风格：
- 线性图标（stroke-based），统一 1.75px 线宽
- 圆角风格，现代科技感
- 蓝色主色调渐变应用图标
- 部分图标添加微妙的填充细节（opacity 0.3-0.4）

更新组件：
- Toolbar: 使用 FolderOpen/Save/SplitView/EditMode/PreviewMode/Moon/Sun
- TabBar: 使用 Close/Plus
- Sidebar: 使用 FolderPlus/File/Folder/ChevronRight
- SearchBar: 使用 ChevronUp/ChevronDown/X
- WelcomeScreen: 使用 AppIcon/WelcomeFile/WelcomeNew
- DropOverlay: 使用 UploadCloud

src/renderer/components/Sidebar/Sidebar.tsx

@@ -2,6 +2,7 @@ import React, { useEffect, useCallback, useState, useRef } from 'react'
 import { useTabStore } from '../../stores/tabStore'
 import { useSidebarStore } from '../../stores/sidebarStore'
 import { getFileName } from '../../lib/fileUtils'
+import { FolderPlus, File, Folder, ChevronRight } from '../Icons'
 import type { FileNode } from '../../types/file'
 
 export function Sidebar() {
@@ -19,22 +20,19 @@ export function Sidebar() {
   const isVisible = useSidebarStore(s => s.isVisible)
   const expandDirs = useSidebarStore(s => s.expandDirs)
 
-  // M-01: 归一化路径分隔符（Windows 混合 \\ 和 /）
+  // M-01: 归一化路径分隔符
   const norm = (p: string) => p.replace(/\\/g, '/')
   const activeFilePath = tabs.find(t => t.id === activeTabId)?.filePath ?? null
 
   // 自动展开到活动文件所在的目录
   useEffect(() => {
     if (!activeFilePath || !rootPath) return
-    // M-01: 归一化后比较
     const normActive = norm(activeFilePath)
     const normRoot = norm(rootPath)
     if (!normActive.startsWith(normRoot)) return
 
-    // 提取文件的所有父目录路径
     const dirsToExpand: string[] = []
-    let dir = activeFilePath.replace(/[/\\][^/\\]+$/, '') // 移除文件名
-    // M-02: 添加 prev 防护无限循环
+    let dir = activeFilePath.replace(/[/\\][^/\\]+$/, '')
     let prev = ''
     while (dir && dir.length >= rootPath.length && dir !== rootPath && dir !== prev) {
       prev = dir
@@ -50,7 +48,6 @@ export function Sidebar() {
   const [isResizing, setIsResizing] = useState(false)
   const sidebarRef = useRef<HTMLDivElement>(null)
 
-  // 打开文件夹
   const handleOpenFolder = useCallback(async () => {
     if (!window.electronAPI) return
     const dirPath = await window.electronAPI.openFolderDialog()
@@ -64,7 +61,6 @@ export function Sidebar() {
     }
   }, [setRootPath, setTree])
 
-  // 刷新目录树
   const refreshTree = useCallback(async () => {
     if (!rootPath || !window.electronAPI) return
     const result = await window.electronAPI.readDirTree(rootPath)
@@ -73,7 +69,6 @@ export function Sidebar() {
     }
   }, [rootPath, setTree])
 
-  // 目录变化监听
   useEffect(() => {
     if (!window.electronAPI) return
     window.electronAPI.onDirChanged(() => {
@@ -84,7 +79,6 @@ export function Sidebar() {
     }
   }, [refreshTree])
 
-  // 侧边栏宽度调节
   useEffect(() => {
     if (!isResizing) return
     const handleMouseMove = (e: MouseEvent) => {
@@ -102,7 +96,6 @@ export function Sidebar() {
     }
   }, [isResizing])
 
-  // L-12: 提取为 useCallback 避免每次渲染重建
   const handleFileClick = useCallback(async (path: string) => {
     const existing = tabs.find(t => t.filePath === path)
     if (existing) {
@@ -114,6 +107,7 @@ export function Sidebar() {
       }
     }
   }, [tabs, switchToTab, createTab])
+
   // M-01: 独立文件区（归一化路径比较）
   const independentFiles = tabs.filter(t => {
     if (!t.filePath) return false
@@ -128,9 +122,7 @@ export function Sidebar() {
       <div id="sidebar-header">
         <span id="sidebar-title">资源管理器</span>
         <button className="sidebar-header-btn" onClick={handleOpenFolder} title="打开文件夹">
-          <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-            <path d="M22 19a2 2 0 0 1-2 2H4a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h5l2 3h9a2 2 0 0 1 2 2z" />
-          </svg>
+          <FolderPlus size={14} />
         </button>
       </div>
 
@@ -147,10 +139,7 @@ export function Sidebar() {
                 onClick={() => switchToTab(tab.id)}
               >
                 <span className="tree-icon">
-                  <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2">
-                    <path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z" />
-                    <polyline points="14 2 14 8 20 8" />
-                  </svg>
+                  <File size={14} />
                 </span>
                 <span className="tree-name">{getFileName(tab.filePath!)}</span>
                 {tab.isModified && <span className="independent-modified-dot"> •</span>}
@@ -176,7 +165,6 @@ export function Sidebar() {
         )}
       </div>
 
-      {/* 调节手柄 */}
       <div
         className="sidebar-resize-handle"
         onMouseDown={() => setIsResizing(true)}
@@ -213,24 +201,17 @@ function FileTree({ nodes, depth, expandedDirs, toggleDir, activeTabId, activeFi
             {node.type === 'dir' ? (
               <>
                 <span className={`tree-arrow ${expandedDirs.has(node.path) ? 'expanded' : ''}`}>
-                  <svg width="10" height="10" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5">
-                    <polyline points="9 18 15 12 9 6" />
-                  </svg>
+                  <ChevronRight size={10} />
                 </span>
                 <span className="tree-icon">
-                  <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2">
-                    <path d="M22 19a2 2 0 0 1-2 2H4a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h5l2 3h9a2 2 0 0 1 2 2z" />
-                  </svg>
+                  <Folder size={14} />
                 </span>
               </>
             ) : (
               <>
                 <span style={{ width: '16px', flexShrink: 0 }} />
                 <span className="tree-icon">
-                  <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2">
-                    <path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z" />
-                    <polyline points="14 2 14 8 20 8" />
-                  </svg>
+                  <File size={14} />
                 </span>
               </>
             )}