feat(groups): show used traffic per group in groups table

Sum up+down across each group's clients via a LEFT JOIN on email in
ListGroups, expose it as trafficUsed on GroupSummary, and render it as a
new column plus a "Total traffic" summary card. Drops the unused "Empty
groups" card and its translation key.

frontend/src/schemas/client.ts

@@ -126,6 +126,7 @@ export const ActiveInboundsByNodeSchema = z
 export const GroupSummarySchema = z.object({
   name: z.string(),
   clientCount: z.number(),
+  trafficUsed: z.number().nullable().transform((v) => v ?? 0),
 });
 
 export const GroupSummaryListSchema = z.array(GroupSummarySchema).nullable().transform((v) => v ?? []);