Title
Feature Request: Support for Personal Server & NAS Backups (WebDAV / SMB / SFTP / HTTPS API)

Body
I would like to suggest adding support for personal server and NAS-based backups in GoodNotes, in addition to the currently supported cloud services.

Many users operate their own servers or NAS solutions (such as Synology, QNAP, or self-hosted servers) for data ownership, privacy, and long-term storage. Supporting standard protocols would allow these users to back up GoodNotes data more flexibly and securely.

Suggested supported protocols include:

WebDAV

SMB (Network Share)

SFTP

HTTPS-based API endpoints

Potential benefits of supporting personal server or NAS backups include:

Greater user control over data ownership and privacy

Reduced reliance on third-party cloud services

Compatibility with enterprise, educational, and self-hosted environments

Seamless integration with existing backup workflows already used by advanced users

This feature could be implemented as:

An advanced or optional backup setting

A manual configuration option where users enter a server address, credentials, and protocol type

A feature clearly labeled for advanced users, minimizing support overhead

For power users, professionals, and institutions that already rely on private infrastructure, personal server and NAS backup support would be a highly valuable addition to GoodNotes.

Thank you for considering this feature request.