fix(backup): paginate cloud backup listings

[zjubiology]

Problem

Cloud storage listings are paginated, but the backup commands only fetched the first page.

This caused three incomplete behaviors when an account had more backups than a single page could contain:

• retention cleanup never examined or deleted expired backups on later pages
• slv backup list only displayed the first page
• the restore prompt could not select backups from later pages

StorageListRes already exposes cursor and truncated, and the storage list and prefix-delete commands already follow the cursor until all pages have been retrieved.

Fix

• add a shared listAllBackups helper that follows cursor while the response is truncated
• use the helper in backup retention cleanup
• use the helper in slv backup list
• use the helper when building the remote restore selection
• preserve the requested prefix and region across every page

Testing

• added a two-page regression test
• verified that the second request receives cursor: "next"
• verified that files from both pages are returned
• verified that prefix and region are preserved between requests