The embedded backup center deep-links *in* fine (mountBackupCenter parses the
sub-segment), but clicking a sub-tab never wrote the URL: BackupPage disables
pushTabToUrl() when embedded and nothing on the fleet side stepped in, so the
address bar stayed stale and the sub-tabs weren't shareable/deep-linkable —
unlike the sibling Migrate sub-tabs.
Give BackupPage an onTabChange hook fired on every switch; the OverviewManager
supplies it to replaceState /apps/overview/backups/<sub> (replaceState, matching
Migrate, so the on-entry deep-link re-sync can't stack a duplicate history
entry). Standalone /backup keeps using pushTabToUrl() unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: librelad <librelad@digitalangels.vip>
73a93cf145