[FEATURE REQUEST] Add WebDAV as a backup target for Package Backup (self-hosted cloud backups)

Start by reading the existing Package Backup and GitHub Cloud Backup implementation, then trace how scheduled and manual backups create the .ubundle/JSON payload and how the Bundle restore flow loads it. The issue describes adding WebDAV configuration, uploads, remote listing and downloads, credential storage, and offline handling; done when those flows work while existing GitHub backup remains unchanged. The payload does not name specific files or tests.

Please confirm these before moving forward.

• I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
• This proposal is a completely new feature. If you want to suggest an improvement or an enhancement, please use this template.

Describe the new feature

Background / Motivation

UniGetUI 3.3.0+ can export the installed-package list as a local .ubundle/JSON bundle and, through Cloud Backup, periodically sync it to a private GitHub repository/Gist tied to the user's GitHub account. This makes reinstalling Windows or migrating to a new PC remarkably easy.

Today GitHub is the only remote backup target. That means users must create and trust a third-party account, depend on its OAuth flow and service availability, and in some regions or restricted networks GitHub access is unreliable or blocked. Users who already run a self-hosted NAS or a WebDAV-capable cloud drive cannot point UniGetUI at storage they own and control. The local export works, but it remains a manual copy step and provides no automatic off-device copy of the backup.

Desired feature

Add "WebDAV" as an optional backup destination alongside the existing GitHub Cloud Backup, under Settings > Package Backup:

• Configuration: server URL (HTTPS), remote folder/path, username and password/app-specific token. Store credentials securely (e.g. Windows Credential Manager / DPAPI), never in plaintext or in logs.
• Reuse the existing backup payload (.ubundle/JSON) and the existing "Periodically perform a backup" schedule plus the manual "Backup now" button: upload the generated file with PUT, while keeping a local copy as a fallback.
• Restore flow: list remote backups (PROPFIND), let the user select one, download it (GET) and feed it into the existing Bundle restore flow.
• Multi-device safety: use per-device filenames (e.g. HOSTNAME\username.ubundle) or explicit naming so two machines do not overwrite each other's backup.
• Robustness: timeouts and retries, clear error reporting that never blocks the local backup, queued uploads while offline, optional timestamped/versioned copies or ETag-based conflict handling, TLS/self-signed certificate options and proxy support.
• GitHub backup stays unchanged and remains the default; WebDAV is opt-in.

Use cases

1. A desktop and a laptop at home/office both push their package lists to one WebDAV share; after a clean Windows install, pick the matching device backup and reinstall everything in a few clicks.
2. Privacy- or compliance-minded users and teams keep their software inventory on an internal NAS reachable only over LAN/VPN, instead of a third-party service.
3. Users of WebDAV-capable drives — Nutstore / 坚果云, Nextcloud, ownCloud, Synology or QNAP NAS, AList — get cloud backup without a GitHub account and without any extra server-side component.
4. Unattended automation: scheduled backups run at logon and upload silently, failed uploads retry later, and the NAS can add its own snapshots/versioning.

Describe how this new feature could help users

Expected benefits

• Cross-device sync and a single central place for all machine backups, with no manual copying.
• Self-hosted data: users decide where backups live, who can read them and how long they are kept; less exposure to third-party account or OAuth changes, service outages, and regional access restrictions.
• Broad compatibility via a standard protocol (RFC 4918): mainstream cloud drives and home/office NAS work out of the box, with no service for UniGetUI to host or maintain.
• Makes scheduled, hands-off off-device backups practical, so the package list survives disk failure, theft, or a system reinstall.
• Additive and low-risk: no change for existing GitHub users; WebDAV client libraries are mature, and the existing backup payload and restore logic can be reused as-is.