fix(i18n): escape '@' in invite email placeholder

vue-i18n treats a bare '@' as the start of a linked-message reference,
so the placeholder "invitee@example.com" failed to compile with
"Invalid linked format". The whole email t-form-item then failed to
render, leaving the invite-member popup with only the role row.

Escape the literal '@' as {'@'} across all four locales so vue-i18n
emits the original text and the email input renders again.

frontend/src/i18n/locales/en-US.ts

@@ -4469,7 +4469,7 @@ export default {
       button: 'Add Member',
       dialogTitle: 'Invite Member',
       emailLabel: 'Email',
-      emailPlaceholder: 'invitee@example.com',
+      emailPlaceholder: "invitee{'@'}example.com",
       roleLabel: 'Role',
       submit: 'Invite',
       success: 'Member added',

frontend/src/i18n/locales/ko-KR.ts

@@ -4529,7 +4529,7 @@ export default {
       button: "멤버 초대",
       dialogTitle: "멤버 초대",
       emailLabel: "이메일",
-      emailPlaceholder: "invitee@example.com",
+      emailPlaceholder: "invitee{'@'}example.com",
       roleLabel: "역할",
       submit: "초대",
       success: "멤버를 추가했습니다",

frontend/src/i18n/locales/ru-RU.ts

@@ -4429,7 +4429,7 @@ export default {
       button: 'Пригласить',
       dialogTitle: 'Пригласить участника',
       emailLabel: 'Email',
-      emailPlaceholder: 'invitee@example.com',
+      emailPlaceholder: "invitee{'@'}example.com",
       roleLabel: 'Роль',
       submit: 'Пригласить',
       success: 'Участник добавлен',

frontend/src/i18n/locales/zh-CN.ts

@@ -4461,7 +4461,7 @@ export default {
       button: "邀请成员",
       dialogTitle: "邀请成员",
       emailLabel: "邮箱",
-      emailPlaceholder: "invitee@example.com",
+      emailPlaceholder: "invitee{'@'}example.com",
       roleLabel: "角色",
       submit: "邀请",
       success: "已添加成员",