feat: P2.3 Folder Sharing + P2.5 Email Import + P2.6 Contact Import + P2.7 Free/Busy

- P2.3: Folder sharing system — ShareFolderDialog, sharing-store, sharing-settings
- P2.5: Email import (.eml, .tgz, .zip) with dedup and progress
- P2.6: Contact import (vCard + CSV) with auto-mapping
- P2.7: Free/Busy view grid with color-coded slots

lib/demo/demo-client.ts

@@ -1037,6 +1037,8 @@ export class DemoJMAPClient implements IJMAPClient {
     return [...this.data.fileNodes];
   }
 
+  async setMailboxShare(): Promise<void> { /* demo: no-op */ }
+
   async setFileNodeShare(): Promise<void> { /* demo: no-op */ }
 
   async getFileNodes(ids: string[] | null): Promise<FileNode[]> {