feat: add OneDrive configuration backup and illustrated setup guide - #760

FluentRead now offers OneDrive in the shared cloud configuration backup entry alongside Google Drive and WebDAV. Users explicitly sign in, preview masked differences, and confirm save, restore or merge; account switching and cancellation preserve the last successful record. The backend uses authorization code + PKCE, app-folder permissions, separate provider state and version checks, with seven interface languages.

Adds an illustrated maintainer tutorial with Microsoft portal screenshots, Chinese arrows, Mermaid diagrams, actual fixture UI screenshots, registration fields, SPA callbacks and build configuration. Public Chinese/English backup and privacy pages explain the new data flow.

Validation:

• 67 cloud-backup tests pass with four-dimensional 100% coverage for the scoped modules.
• Type checking, Chrome/Firefox, userscript and documentation builds, manifest checks and static website validation pass.
• 23 isolated Edge assertions verify credential restoration, account switching, cancellation, failure feedback, provider isolation and English dark mobile layout using synthetic OAuth/Graph responses.
• Four existing architecture failures reproduce on the original clean baseline; evidence is recorded in docs/maintainers/reports/onedrive-sync-20261003.

A real Microsoft application has not been registered yet. Builds without a valid WXT_ONEDRIVE_CLIENT_ID show an unavailable explanation. Real personal/organization accounts, registered callbacks and cross-device restoration require the tutorial's live acceptance checks before enabling this in a release. The fixed public application encryption passphrase is not a user-exclusive secret, and uploads do not claim cross-device atomicity.