refactor: tmpo backup list now displays backups in chronological order

[DylanDevelops]

Pull Request Checklist

• I have read and followed the contribution guidelines.
• My pull request targets the main branch of tmpo.
• I have tested these changes locally on my machine.

Description

This pull request updates the backup listing functionality to sort and display backups in oldest-to-newest order (instead of newest-to-oldest), and updates the CLI output to clearly indicate the latest backup. The test suite and documentation are also updated to match this new behavior.

Backup ordering and display changes:

• Changed the ListBackups function in internal/storage/backup.go to return backups sorted oldest-first (1 = oldest), instead of newest-first. This includes updating the sorting logic and the function's documentation comment. [1] [2]
• Updated the CLI output in cmd/backups/list.go to add a "← latest" tag next to the most recent (latest) backup, making it clear to users which backup is the newest.

Test and documentation updates:

• Renamed and updated the test TestListBackups_SortedNewestFirst to TestListBackups_SortedOldestFirst in internal/storage/backup_test.go, and adjusted assertions to expect oldest-first order. [1] [2]