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.

internal/web/translation/ar-EG.json

@@ -934,8 +934,8 @@
       "deleteSuccess": "تم مسح المجموعة من {count} عميل.",
       "resetTraffic": "إعادة تعيين حركة المرور",
       "resetConfirmTitle": "إعادة تعيين حركة المرور للمجموعة {name}؟",
-      "resetConfirmContent": "يصفر up/down لجميع {count} عميل في هذه المجموعة.",
-      "resetSuccess": "تمت إعادة تعيين حركة المرور لـ {count} عميل.",
+      "resetConfirmContent": "يعيد تعيين عداد حركة مرور المجموعة فقط؛ ولا تتأثر عدادات العملاء الفرديين.",
+      "resetSuccess": "تمت إعادة تعيين حركة مرور المجموعة {name}.",
       "adjustSuccess": "تم ضبط {count} عميل في {name}.",
       "emptyForAction": "هذه المجموعة فارغة.",
       "deleteGroupOnly": "حذف المجموعة (مع الاحتفاظ بالعملاء)",