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/SearchBar/SearchBar.tsx

@@ -2,6 +2,7 @@ import React, { useCallback, useRef, useEffect } from 'react'
 import { useSearchStore } from '../../stores/searchStore'
 import { useTabStore } from '../../stores/tabStore'
 import { findMatches } from '../../lib/searchEngine'
+import { ChevronUp, ChevronDown, X } from '../Icons'
 
 export function SearchBar() {
   const searchInputRef = useRef<HTMLInputElement>(null)
@@ -25,7 +26,6 @@ export function SearchBar() {
 
   const getActiveTab = useTabStore(s => s.getActiveTab)
 
-  // 执行搜索
   const doSearch = useCallback((text: string) => {
     const tab = getActiveTab()
     if (!tab || !text) {
@@ -36,21 +36,18 @@ export function SearchBar() {
     setMatches(found)
   }, [getActiveTab, options, setMatches])
 
-  // 搜索文本变化
   const handleSearchChange = useCallback((e: React.ChangeEvent<HTMLInputElement>) => {
     const text = e.target.value
     setSearchText(text)
     doSearch(text)
   }, [setSearchText, doSearch])
 
-  // 替换当前
   const handleReplaceCurrent = useCallback(() => {
     const tab = getActiveTab()
     if (!tab || matches.length === 0 || currentIndex < 0) return
     const m = matches[currentIndex]
     const newContent = tab.content.substring(0, m.start) + replaceText + tab.content.substring(m.end)
     useTabStore.getState().updateTabContent(tab.id, newContent)
-    // 重新搜索
     doSearch(searchText)
   }, [getActiveTab, matches, currentIndex, replaceText, searchText, doSearch])
 
@@ -67,7 +64,6 @@ export function SearchBar() {
     doSearch(searchText)
   }, [getActiveTab, matches, replaceText, searchText, doSearch])
 
-  // 打开时聚焦
   useEffect(() => {
     if (isVisible && searchInputRef.current) {
       searchInputRef.current.focus()
@@ -123,16 +119,14 @@ export function SearchBar() {
           .*
         </button>
         <button className="search-nav-btn" onClick={findPrev} title="上一个 (Shift+Enter)">
-          <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5">
-            <polyline points="18 15 12 9 6 15" />
-          </svg>
+          <ChevronUp size={12} />
         </button>
         <button className="search-nav-btn" onClick={findNext} title="下一个 (Enter)">
-          <svg width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2.5">
-            <polyline points="6 9 12 15 18 9" />
-          </svg>
+          <ChevronDown size={12} />
+        </button>
+        <button className="search-nav-btn" onClick={close} title="关闭 (Escape)">
+          <X size={14} />
         </button>
-        <button className="search-nav-btn" onClick={close} title="关闭 (Escape)">✕</button>
       </div>
 
       {showReplace && (