feat(groups): 添加从其他分组复制账号功能

- 创建分组时可选择从已有分组复制账号
- 编辑分组时支持同步账号（全量替换操作）
- 仅允许选择相同平台的源分组
- 添加完整的数据校验：去重、自引用检查、平台一致性检查
- 前端支持多选源分组，带提示说明操作行为

frontend/src/i18n/locales/en.ts

@@ -1004,6 +1004,14 @@ export default {
         fallbackHint: 'Non-Claude Code requests will use this group. Leave empty to reject directly.',
         noFallback: 'No Fallback (Reject)'
       },
+      copyAccounts: {
+        title: 'Copy Accounts from Groups',
+        tooltip: 'Select one or more groups of the same platform. After creation, all accounts from these groups will be automatically bound to the new group (deduplicated).',
+        tooltipEdit: 'Select one or more groups of the same platform. After saving, current group accounts will be replaced with accounts from these groups (deduplicated).',
+        selectPlaceholder: 'Select groups to copy accounts from...',
+        hint: 'Multiple groups can be selected, accounts will be deduplicated',
+        hintEdit: '⚠️ Warning: This will replace all existing account bindings'
+      },
       modelRouting: {
         title: 'Model Routing',
         tooltip: 'Configure specific model requests to be routed to designated accounts. Supports wildcard matching, e.g., claude-opus-* matches all opus models.',