Empty a folder in batches the server will accept

Emptying Deleted Items back-referenced one Email/query straight into one
Email/set, so every id in the folder arrived in a single call. Stalwart
refuses the whole call over maxObjectsInSet with requestTooLarge, which
left a folder of 5192 messages impossible to empty at all.

Walk the folder a page at a time instead: the filter re-runs each pass,
so the next page is whatever is still there. A pass that destroys nothing
stops the loop and reports the server's error rather than spinning.

The same defect sat in two neighbours. Delete-forever on a large
selection sent every id in one Email/set, and mark-all-read fed up to
5000 ids into an Email/get that only echoed them back - past
maxObjectsInGet, and for no gain, since the query already returned them.
Both now page through the same ceiling, read from the session rather than
hardcoded.

The mock advertised maxObjectsInSet but never enforced it, so none of
this could fail in a test. It now rejects oversized get and set calls the
way Stalwart does.

While here, restrict emptying to Deleted Items. It was offered on Junk
too, where a permanent one-shot clear is harder to justify; Junk is now
select-all plus Delete, which takes the batched path.

web/src/views/mail/MailboxTree.tsx

@@ -261,7 +261,7 @@ function MailboxMenu({ mailbox: m, onCreateChild, onShare }: { mailbox: Mailbox;
       <MenuItem icon={m.isSubscribed ? <EyeOff size={16} /> : <Eye size={16} />} label={m.isSubscribed ? "Hide from list" : "Show in list"} onClick={() => void useMail.getState().updateMailbox(m.id, { isSubscribed: !m.isSubscribed })} disabled={m.role === "inbox"} />
       <MenuItem icon={<Share2 size={16} />} label="Share…" onClick={onShare} />
       <MenuSep />
-      {(m.role === "trash" || m.role === "junk") && <MenuItem icon={<Eraser size={16} />} label="Empty folder" onClick={() => void empty()} danger />}
+      {m.role === "trash" && <MenuItem icon={<Eraser size={16} />} label="Empty folder" onClick={() => void empty()} danger />}
       <MenuItem icon={<Trash2 size={16} />} label="Delete folder" onClick={() => void remove()} danger disabled={isSpecial || !m.myRights.mayDelete} />
     </>
   );