fix(users): every user row shows its avatar, not only its initials

System Health's per-user CPU/RAM card asked UserAvatar for the viewer's own
picture with no user id, and the host stats never carried avatarAt, so every
card fell back to letters. The groups now carry avatarAt and the card passes
userId. The user editor's header, the pool board's member chips and the pool
member picker drew hand-made initials; they use UserAvatar too.

client/src/components/users/UserAvatar.tsx

@@ -4,7 +4,9 @@
  * (`GET /api/me/avatar`) or, with `userId`, another's (`GET
  * /api/users/:id/avatar`, staff only), versioned by `avatarAt`; without one —
  * or when the image fails to load — the first letters of its name. Used by the
- * sidebar's account row, its workspace owner groups and Settings → Account.
+ * sidebar's account row, its workspace owner groups, Settings → Account, the
+ * Users page and its editor, the pool board and member picker, and System
+ * Health's per-user CPU/RAM card.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */