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/Toolbar/Toolbar.tsx

@@ -1,4 +1,5 @@
 import React from 'react'
+import { FolderOpen, Save, SplitView, EditMode, PreviewMode, Moon, Sun } from '../Icons'
 
 interface ToolbarProps {
   onOpen: () => void
@@ -14,61 +15,30 @@ export function Toolbar({ onOpen, onSave, viewMode, onViewModeChange, darkMode,
     <div id="toolbar">
       <div className="toolbar-left">
         <button className="toolbar-btn" onClick={onOpen} title="打开文件 (Ctrl+O)">
-          <svg width="18" height="18" 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>
+          <FolderOpen size={18} />
           <span>打开</span>
         </button>
         <button className="toolbar-btn" onClick={onSave} title="保存文件 (Ctrl+S)">
-          <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-            <path d="M19 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h11l5 5v11a2 2 0 0 1-2 2z" />
-            <polyline points="17 21 17 13 7 13 7 21" />
-            <polyline points="7 3 7 8 15 8" />
-          </svg>
+          <Save size={18} />
           <span>保存</span>
         </button>
         <div className="toolbar-divider" />
         <button className={`toolbar-btn ${viewMode === 'split' ? 'active' : ''}`} onClick={() => onViewModeChange('split')} title="编辑+预览 (Ctrl+1)">
-          <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-            <rect x="3" y="3" width="18" height="18" rx="2" ry="2" />
-            <line x1="12" y1="3" x2="12" y2="21" />
-          </svg>
+          <SplitView size={18} />
           <span>分屏</span>
         </button>
         <button className={`toolbar-btn ${viewMode === 'editor' ? 'active' : ''}`} onClick={() => onViewModeChange('editor')} title="纯编辑 (Ctrl+2)">
-          <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-            <path d="M11 4H4a2 2 0 0 0-2 2v14a2 2 0 0 0 2 2h14a2 2 0 0 0 2-2v-7" />
-            <path d="M18.5 2.5a2.121 2.121 0 0 1 3 3L12 15l-4 1 1-4 9.5-9.5z" />
-          </svg>
+          <EditMode size={18} />
           <span>编辑</span>
         </button>
         <button className={`toolbar-btn ${viewMode === 'preview' ? 'active' : ''}`} onClick={() => onViewModeChange('preview')} title="纯预览 (Ctrl+3)">
-          <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-            <path d="M1 12s4-8 11-8 11 8 11 8-4 8-11 8-11-8-11-8z" />
-            <circle cx="12" cy="12" r="3" />
-          </svg>
+          <PreviewMode size={18} />
           <span>预览</span>
         </button>
       </div>
       <div className="toolbar-right">
         <button className="toolbar-btn" onClick={onToggleDark} title="切换暗色主题">
-          {darkMode ? (
-            <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-              <circle cx="12" cy="12" r="5" />
-              <line x1="12" y1="1" x2="12" y2="3" />
-              <line x1="12" y1="21" x2="12" y2="23" />
-              <line x1="4.22" y1="4.22" x2="5.64" y2="5.64" />
-              <line x1="18.36" y1="18.36" x2="19.78" y2="19.78" />
-              <line x1="1" y1="12" x2="3" y2="12" />
-              <line x1="21" y1="12" x2="23" y2="12" />
-              <line x1="4.22" y1="19.78" x2="5.64" y2="18.36" />
-              <line x1="18.36" y1="5.64" x2="19.78" y2="4.22" />
-            </svg>
-          ) : (
-            <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
-              <path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z" />
-            </svg>
-          )}
+          {darkMode ? <Sun size={18} /> : <Moon size={18} />}
         </button>
       </div>
     </div>