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/lib/api.ts

@@ -684,6 +684,8 @@ export interface HostUsageGroup {
   username?: string;
   /** The user's display name, null when none is set (absent from an older server). */
   displayName?: string | null;
+  /** When the user's avatar was last set (null = none; absent from an older server). */
+  avatarAt?: string | null;
   /** % of the whole machine's CPU (100 = every core busy) over the sample. */
   cpu: number;
   /** Resident memory, bytes (shared pages count in every process). */