v1.0.1 - WordPress.org Review Fixes

Fixes for WordPress.org plugin review (Review ID: R zen-backup/themexplosion/18May26/T2 22May26/4.0.1B1)

• All file storage moved from wp-content/zenbu to wp_upload_dir()/zen-backup/
• Manifests, backups, lock files, and temp chunks now stored under uploads/zen-backup/
• Activation hook migrates legacy data from old locations
• .htaccess and index.html guards protect uploads/zen-backup/ from direct access