feat(groups): reset group traffic without touching client counters

The group page shows traffic counting per group, but the only reset
available zeroed every member client's up/down counters (and their
quotas) via bulkResetTraffic. Group traffic is a derived sum of client
traffic, so zeroing the group display previously required mutating the
clients themselves.

Add a display-only baseline: ClientGroup gains reset_up/reset_down
columns (additive, handled by AutoMigrate). ResetGroupTraffic snapshots
the group's current up/down sum into the baseline, and ListGroups now
reports max(0, sum - baseline). Client counters are left untouched and
no Xray restart is triggered. A new POST /panel/api/clients/groups/
resetTraffic endpoint drives it, creating the client_groups row when the
group exists only as a derived label.

The groups page action now calls the new endpoint; confirm/success
strings updated across all 13 locales to reflect group-only semantics.

frontend/src/pages/api-docs/endpoints.ts

@@ -784,6 +784,13 @@ export const sections: readonly Section[] = [
         body: '{\n  "name": "customer-a"\n}',
         response: '{\n  "success": true,\n  "obj": {\n    "affected": 5\n  }\n}',
       },
+      {
+        method: 'POST',
+        path: '/panel/api/clients/groups/resetTraffic',
+        summary: 'Reset only the group-level traffic counter shown on the groups page. Snapshots the current up/down sum of the group\'s members as a baseline so the group total reads zero, while leaving each client\'s own counters (and their quotas) untouched. No Xray restart is triggered. Creates the client_groups row if the group exists only as a derived label.',
+        body: '{\n  "name": "customer-a"\n}',
+        response: '{\n  "success": true,\n  "obj": {\n    "name": "customer-a"\n  }\n}',
+      },
       {
         method: 'POST',
         path: '/panel/api/clients/resetTraffic/:email',