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/mail/services/dto/mail.ts

@@ -0,0 +1,122 @@
+// Lecture de la configuration mail (singleton admin)
+export type MailConfigurationDto = {
+    protocol: string | null
+    imapHost: string | null
+    imapPort: number | null
+    imapEncryption: string | null
+    smtpHost: string | null
+    smtpPort: number | null
+    smtpEncryption: string | null
+    username: string | null
+    sentFolderPath: string | null
+    enabled: boolean
+    hasPassword: boolean
+    // password JAMAIS présent dans les réponses GET
+}
+
+// Input PATCH configuration (password optionnel, write-only)
+export type MailConfigurationUpdateDto = {
+    protocol?: string | null
+    imapHost?: string | null
+    imapPort?: number | null
+    imapEncryption?: string | null
+    smtpHost?: string | null
+    smtpPort?: number | null
+    smtpEncryption?: string | null
+    username?: string | null
+    sentFolderPath?: string | null
+    enabled?: boolean
+    password?: string       // write-only, jamais retourné
+}
+
+// Résultat du test de connexion
+export type MailTestConnectionResultDto = {
+    ok: boolean
+    foldersCount?: number
+    error?: string
+}
+
+// Dossier mail (peut être imbriqué)
+export type MailFolderDto = {
+    path: string
+    displayName: string
+    parentPath: string | null
+    unreadCount: number
+    totalCount: number
+    children?: MailFolderDto[]
+}
+
+// Adresse mail (nom + email)
+export type MailAddressDto = {
+    name: string | null
+    email: string
+}
+
+// En-tête d'un message (liste)
+export type MailMessageHeaderDto = {
+    id: number
+    messageId: string       // identifiant IMAP unique
+    folderPath: string
+    subject: string | null
+    fromName: string | null
+    fromEmail: string | null
+    toRecipients: MailAddressDto[]
+    ccRecipients: MailAddressDto[]
+    sentAt: string | null   // ISO 8601
+    receivedAt: string      // ISO 8601
+    isRead: boolean
+    isFlagged: boolean
+    hasAttachments: boolean
+    linkedTaskIds: number[]
+}
+
+// Pièce jointe (métadonnées uniquement, téléchargement via downloadId)
+export type MailAttachmentDto = {
+    downloadId: string
+    filename: string
+    mimeType: string
+    size: number            // octets
+}
+
+// Détail complet d'un message (enrichi avec body + PJ)
+export type MailMessageDetailDto = {
+    header: MailMessageHeaderDto
+    bodyHtml: string | null    // HTML brut — TOUJOURS passer par sanitizeMailHtml() avant affichage
+    bodyText: string | null    // Fallback texte plain
+    attachments: MailAttachmentDto[]
+}
+
+// Page de messages paginée (cursor-based)
+export type MailMessagesPageDto = {
+    items: MailMessageHeaderDto[]
+    nextCursor: string | null   // null = plus de page suivante
+    total: number
+}
+
+// Input : marquer lu/non-lu
+export type MailMessageReadInput = {
+    read: boolean
+}
+
+// Input : marquer étoilé/non-étoilé
+export type MailMessageFlagInput = {
+    flagged: boolean
+}
+
+// Input : créer une tâche depuis un mail
+export type MailCreateTaskInput = {
+    projectId: number
+    taskGroupId?: number | null
+    assigneeId?: number
+    statusId?: number
+}
+
+// Input : lier une tâche existante à un mail
+export type MailLinkTaskInput = {
+    taskId: number
+}
+
+// Résultat de la sync manuelle
+export type MailSyncResultDto = {
+    dispatched: boolean
+}