members: show joined date in admin + public lists; 'last active: X ago'

Admin Users view appends 'joined YYYY-MM-DD'. Public member directory
shows 'last active: N days ago' for offline members plus the joined date,
both sourced from store.User.CreatedAt.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/admin/admin.go

@@ -344,6 +344,9 @@ func (m Model) viewUsers() (string, string) {
 		if u.EmailVerified {
 			flags = append(flags, "verified")
 		}
+		if !u.CreatedAt.IsZero() {
+			flags = append(flags, "joined "+u.CreatedAt.Format("2006-01-02"))
+		}
 		name := u.Name
 		if u.Banned {
 			name = warnStyle.Render(name + " [BANNED]")