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/components/MailRefreshButton.vue

@@ -0,0 +1,24 @@
+<script setup lang="ts">
+import { useMailStore } from '~/modules/mail/stores/mail'
+
+const store = useMailStore()
+const { syncing } = storeToRefs(store)
+
+const { t } = useI18n()
+
+async function handleRefresh(): Promise<void> {
+    await store.triggerSync()
+}
+</script>
+
+<template>
+    <MalioButton
+        :label="t('mail.actions.refresh')"
+        variant="secondary"
+        icon-name="material-symbols:refresh"
+        icon-position="left"
+        :icon-size="16"
+        :disabled="syncing"
+        @click="handleRefresh"
+    />
+</template>