Use DataTable for members, add seed users, remove teams stats

- Update TeamMembersPage to use DataTable component for consistency
- Add test users (alice, bob, charlie, diana, eve, frank) with various roles
- Remove stats from teams list header
- Passwords for test users are same as their usernames

frontend/src/pages/TeamMembersPage.css

@@ -16,34 +16,11 @@
   font-size: 1.75rem;
 }
 
-/* Members list */
-.members-list {
-  display: flex;
-  flex-direction: column;
-  gap: 0.5rem;
-}
-
-.member-card {
-  display: flex;
-  justify-content: space-between;
-  align-items: center;
-  padding: 1rem;
-  background: var(--color-bg-secondary);
-  border: 1px solid var(--color-border);
-  border-radius: var(--radius-md);
-  gap: 1rem;
-}
-
-.member-card.current-user {
-  background: var(--color-primary-bg);
-  border-color: var(--color-primary-border, var(--color-border));
-}
-
-.member-info {
+/* Member cell in table */
+.member-cell {
   display: flex;
   align-items: center;
   gap: 0.75rem;
-  min-width: 0;
 }
 
 .member-avatar {
@@ -87,11 +64,8 @@
   white-space: nowrap;
 }
 
-.member-actions {
-  display: flex;
-  align-items: center;
-  gap: 0.5rem;
-  flex-shrink: 0;
+.text-muted {
+  color: var(--color-text-muted);
 }
 
 .role-select {