feat(mailing-lists): add Aliases count column

Same account-alias-count-column deviation already used on Accounts and
Groups, extended to x:MailingList: adds a synthetic aliasCount column
and lets DynamicList's existing generic column-name-driven handling
resolve/render it — no changes needed there, the mechanism was already
list-agnostic.

Verified in the running dev server: the column renders 0 with no
aliases set on either seeded mailing list.

src/lib/mailingListColumns.ts

@@ -0,0 +1,28 @@
+/*
+ * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <hello@stalw.art>
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ */
+
+import type { Schema } from '@/types/schema';
+
+/**
+ * SCHEMA-DEVIATION: account-alias-count-column (see SCHEMA_DEVIATIONS.md)
+ *
+ * The Mailing Lists list doesn't expose alias count as a column, only the
+ * full `aliases` objectList on the detail view. Adds the same synthetic
+ * `aliasCount` column already used on Accounts/Groups — DynamicList's
+ * generic count-column handling picks it up with no further wiring.
+ */
+export function withMailingListColumns(schema: Schema): Schema {
+  const list = schema.lists['x:MailingList'];
+  if (!list) return schema;
+
+  return {
+    ...schema,
+    lists: {
+      ...schema.lists,
+      'x:MailingList': { ...list, columns: [...list.columns, { name: 'aliasCount', label: 'Aliases' }] },
+    },
+  };
+}