Add B2-to-B2 sync between buckets and prefixes

[goanpeca]

Summary

The SDK sync package supports remote-to-remote synchronization, but the extension issues currently focus on local-to-B2 and B2-to-local workflows. B2-to-B2 sync is useful for account cleanup, migrations, prefix mirroring, and staging/production bucket workflows.

SDK capability

• synchronize(...) from @backblaze-labs/b2-sdk/sync
• B2Folder to represent source and destination prefixes
• Sync actions including copy, hide, delete, and dry-run planning

Proposed UI

Add a sync wizard that can choose a source bucket/prefix and destination bucket/prefix, preview the planned actions, then run the sync with progress. This should be distinct from the local-folder sync tracked in #72.

Acceptance criteria

• Users can select source and destination B2 bucket/prefix pairs.
• The sync can run in dry-run mode and show planned changes before execution.
• Users can choose compare/keep behavior supported by the SDK.
• Progress and errors are visible in VS Code.
• Tests cover direction selection and action-plan execution plumbing.

Related

• Local to B2 sync (mirror a workspace folder to or from a bucket) #72 tracks local workspace folder sync to/from B2.