mgr/dashboard: improve cephfs mirror peer summary

- Aggregate per-daemon peer stats across mirror daemons
- Sum sync_bytes across mirrored paths for total bytes
- Use sync_time_stamp epoch for last sync display
- Rename daemon-status endpoint to mirror/daemon/status

Assisted-by: Claude Sonnet 5
Fixes: https://tracker.ceph.com/issues/79116
Signed-off-by: Pedro Gonzalez Gomez <pegonzal@ibm.com>