Ask for shareWith on mailboxes too

The third store fetching everything by asking for nothing. Same cause as
the calendars and address books a commit ago: Stalwart does not return
`shareWith` unless a client names it, so mail folders never looked shared
either.

This one has a narrow but real consequence. Sharing a mail folder is
withdrawn, because Stalwart stores the share and never delivers it, and
the only way left to clear one already made is the "Stop sharing" entry
-- which appears only when a folder looks shared. Without the property it
never did. The escape hatch built for exactly that situation could not be
reached from the situation it was built for.

Found by looking for the rest of them rather than waiting for the next
report: `ids: null` with no `properties`, across the app. The others it
turned up -- Sieve scripts, identities, the vacation response, quotas,
participant identities, push subscriptions -- have no `shareWith` to
lose, so mailboxes were the last.

The mock hides it here as well now, so all three are honest.

server/src/mock/index.ts

@@ -470,7 +470,7 @@ const handlers: Record<string, Handler> = {
     const list = ids.filter((id) => id === ACCOUNT).map((id) => ({ id, name: USER, locale: MOCK_LOCALE, timeZone: null }));
     return { accountId: ACCOUNT, state: String(state.n), list, notFound: ids.filter((id) => id !== ACCOUNT) };
   },
-  "Mailbox/get": genericGet(mailboxes),
+  "Mailbox/get": (a) => hideShareWithUnlessAsked(a, genericGet(mailboxes)(a) as { list: Obj[] }) as never,
   "Mailbox/set": (a) => { const r = genericSet(mailboxes, "m", (o) => Object.assign(o, { ...mb(o.id as string, o.name as string, null, (o.parentId as string) ?? null), ...o }))(a); recount(); return r; },
   "Mailbox/changes": () => ({ accountId: ACCOUNT, oldState: "1", newState: String(state.n), hasMoreChanges: false, created: [], updated: [], destroyed: [] }),
   "Email/query": (a) => {