chore(i18n): pt-BR + en keys for activity, costsSection, logsGroup, systemGroup, costsOverview (B/F3)

Add new sidebar i18n keys without removing existing ones (back-compat B11/B12):
- sidebar.activity / sidebar.activitySubtitle
- sidebar.logsGroup
- sidebar.systemGroup
- sidebar.costsOverview / sidebar.costsOverviewSubtitle
Existing sidebar.costs, sidebar.costsSection, sidebar.costsSubtitle preserved.

src/i18n/messages/en.json

@@ -930,7 +930,13 @@
     "settingsAuthzSubtitle": "Route inventory and bypass policy",
     "docsSubtitle": "Documentation",
     "issuesSubtitle": "Report a bug",
-    "changelogSubtitle": "Release notes"
+    "changelogSubtitle": "Release notes",
+    "activity": "Activity",
+    "activitySubtitle": "Friendly feed of recent events",
+    "logsGroup": "Logs",
+    "systemGroup": "System",
+    "costsOverview": "Overview",
+    "costsOverviewSubtitle": "Consolidated cost analysis"
   },
   "webhooks": {
     "title": "Webhooks",

src/i18n/messages/pt-BR.json

@@ -930,7 +930,13 @@
     "settingsAuthzSubtitle": "Inventário de rotas e política de bypass",
     "docsSubtitle": "Documentação",
     "issuesSubtitle": "Reportar um bug",
-    "changelogSubtitle": "Notas de versão"
+    "changelogSubtitle": "Notas de versão",
+    "activity": "Atividade",
+    "activitySubtitle": "Feed amigável de eventos recentes",
+    "logsGroup": "Logs",
+    "systemGroup": "Sistema",
+    "costsOverview": "Visão geral",
+    "costsOverviewSubtitle": "Análise consolidada de custos"
   },
   "webhooks": {
     "title": "Webhooks",