feat: 导入改用原生对话框、新增 app-store 单元测试、修复窗口滚动

- handleImport 从 DOM <input> hack 改为 @tauri-apps/plugin-dialog 原生文件选择
- 新增 Rust read_text_file 命令读取文件内容，零外部依赖
- 新增 tests/unit/app-store.test.ts，25 个测试覆盖 CRUD/undo-redo/loadPaths/savePaths
- 修复 AppShell overflow-hidden 导致无法滚动窗口的问题

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/components/layout/AppShell.tsx

@@ -84,7 +84,7 @@ export function AppShell() {
       </div>
 
       <div
-        className="flex-1 overflow-hidden"
+        className="flex-1 overflow-auto"
         onDragOver={(e) => { e.preventDefault(); e.dataTransfer.dropEffect = 'link'; }}
         onDrop={(e) => {
           e.preventDefault();