"Mine" filter shows external storage files as owned by everyone

External storage mounts (S3, SMB, local, etc.) report the current session user as owner-id regardless of real ownership, and the "Mine" filter in OfficeOverview.vue only excludes group/shared mount-types — so external slips through and those files show as "Mine" for every user who has the mount. Confirmed the actual affected mount is S3-backed external storage.

Reproduced locally (with a local backend mount, since ExternalMountPoint::getMountType() returns the same external value regardless of storage backend — S3, SMB, or local all hit the same code path): the same file on a shared external storage mount returns owner-id: admin when queried as admin and owner-id: alice when queried as alice.