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/app/layouts/default.vue

@@ -139,15 +139,20 @@ const route = useRoute()
 const { t } = useI18n()
 const { sections } = useSidebar()
 
+// `/mail` est déclaré dans config/sidebar.php pour le gating module (disabledRoutes),
+// mais son rendu visuel + badge non-lus est géré manuellement ci-dessous (feature-flag Mail).
+// On le filtre des sections dynamiques pour éviter un doublon dans la nav.
 const translatedSections = computed(() =>
     sections.value.map((section) => ({
         label: t(section.label),
         icon: section.icon,
-        items: section.items.map((item) => ({
-            label: t(item.label),
-            to: item.to,
-            icon: item.icon,
-        })),
+        items: section.items
+            .filter((item) => item.to !== '/mail')
+            .map((item) => ({
+                label: t(item.label),
+                to: item.to,
+                icon: item.icon,
+            })),
     })),
 )