feat(mail) : extract Mail front into Nuxt module layer

LST-67 (2.5) front. Completes the Mail module.

- New frontend/modules/mail/ layer (auto-detected): /mail page (3 columns),
  7 components, mail service + DTO, mail store (folders/messages/unread polling).
- sanitizeMailHtml util and useSystemFolderLabel composable stay global;
  AdminMailTab stays in /admin (service import repointed).
- Consumers repointed: AdminMailTab and PM TaskModal -> ~/modules/mail/...;
  the store is auto-imported (Pinia storesDirs) so the layout badge/polling is
  unchanged.
- /mail gated by the mail module: sidebar.php item with module=mail (so
  SidebarFilter disables /mail when the module is off); the layout filters /mail
  from the API sections to avoid a visual duplicate. ROLE_CLIENT exclusion kept.
- i18n key sidebar.general.mail added.

nuxt build passes; /mail and all other routes preserved.

frontend/modules/project-management/components/TaskModal.vue

@@ -551,8 +551,8 @@ import { useTaskService } from '~/modules/project-management/services/tasks'
 import { useTaskRecurrenceService } from '~/modules/project-management/services/task-recurrences'
 
 import type { Project } from '~/modules/project-management/services/dto/project'
-import { useMailService } from '~/services/mail'
-import type { MailMessageHeaderDto } from '~/services/dto/mail'
+import { useMailService } from '~/modules/mail/services/mail'
+import type { MailMessageHeaderDto } from '~/modules/mail/services/dto/mail'
 
 const props = defineProps<{
     modelValue: boolean