feat(viewer): Add data source dialog

Introduce a dialog for switching between built-in content and a
user-selected local folder. Persist the folder handle in IndexedDB
so the selection survives page refreshes. Scan the folder for .md,
.yarn, and .csv files to build the file index.

Update App and Sidebar to pass file tree and heading data, and
add TypeScript declarations for the File System Access API.

src/components/index.ts

@@ -21,6 +21,8 @@ export type { SidebarProps } from "./Sidebar";
 export { FileTreeNode, HeadingNode } from "./FileTree";
 export { default as DocDialog } from "./DocDialog";
 export type { DocDialogProps } from "./DocDialog";
+export { default as DataSourceDialog } from "./DataSourceDialog";
+export type { DataSourceDialogProps } from "./DataSourceDialog";
 
 // 导出数据类型
 export type { DiceProps } from "./md-dice";