[Feature Proposal] OneDrive cloud sync backend (companion to google_drive.c)

Cloud Sync recently gained a Google Drive backend (network/cloud_sync/google_drive.c, Feb 2026) alongside WebDAV/S3/SMB/iCloud, and #17972 unlocked SSL-backed sync on Android. OneDrive is one of the largest consumer clouds and currently has no backend — OneDrive users are the biggest unserved group left.

Proposal: network/cloud_sync/onedrive.c, structured directly on google_drive.c:

• Microsoft identity platform device-code flow — public client, notably no client secret needed (simpler than the Google flow)
• refresh token persisted in settings, same pattern as google_drive_refresh_token
• storage via Microsoft Graph app folder (/me/drive/special/approot) so RetroArch only sees its own folder; simple PUT …/content uploads (save files are small; upload sessions only needed >4 MB)
• same 5-function cloud_sync_driver_t vtable, C89, -Wall clean, house style

Questions before any code is written:

1. Is this a backend the team wants?
2. Who registers/owns the (free) Azure app client_id for official builds? A fork can use a personal registration for development, but production should be libretro-owned, like the Google credentials.
3. Any preference on app-folder scope (Files.ReadWrite.AppFolder) vs. broader drive access?

I can have the implementation contributed and tested on Android + Windows desktop. Transparency note: the implementation would be developed with AI assistance (Claude), with human review and on-device testing — flagging this up front; happy to discuss.

CC @warmenhoven (author of the webdav/gdrive/s3 backends and the Android TLS work). Related: #6875 (native cloud saving bounty). While testing the gdrive backend I also filed #19244 (folder-creation race) — happy to help verify fixes there too.

Start with network/cloud_sync/google_drive.c and the existing cloud-sync backends, then review the five-function cloud_sync_driver_t interface and the google_drive_refresh_token settings pattern. Done means an approved design and a tested network/cloud_sync/onedrive.c backend using the agreed Azure app ownership and folder scope, verified on Android and Windows desktop.