Stop offering to share mail folders, and let a share be removed

Sharing a mail folder does nothing. `Mailbox/set` takes the `shareWith`
map, `Mailbox/get` reads it back, and the folder never appears for the
account it was shared with -- confirmed on the live 0.16.19 with a folder
shared read-only to another account on the same server, which never saw
it. Stalwart's sharing documentation lists calendars, address books and
file storage; mail folders are not among them. Nothing anywhere reports a
failure, so a client that trusts what it reads back shows the share as
live for ever, which is what happened.

The entry point is withdrawn. Address book sharing goes with it on a
report that it behaved the same way -- not reproduced, and contradicted
by Stalwart's own docs, so that one is expected back; it is out because
offering a share nobody can verify was worse than the gap. Files and
calendars are untouched.

Removing a share was impossible, for a reason worth writing down. The
dialog rendered the list of who a thing was shared with *inside* the
branch that runs when the directory has principals to offer. A server
with `allowDirectoryQueries` off returns none -- that is the default, and
it is how these shares came to be made in the first place -- so the
dialog showed one line of hint and nothing else. The share was there, and
there was no way to see it, let alone remove it. The list is now rendered
whatever the directory says; only the control for adding somebody new
depends on having somebody to add.

So the withdrawn entry points do not strand what they created: a folder
or book already shared still offers "Stop sharing", which is the one
thing you want when the share is invisible everywhere else.

The API was never the problem, which is worth recording since it was the
first guess: `shareWith: null` is accepted and clears the map, tested
against the live server on the stuck folder, which is now unshared.

web/src/views/mail/MailboxTree.tsx

@@ -292,6 +292,7 @@ function FolderRow({ mailbox: m, label, depth, hasChildren, open, hiddenUnread,
 }
 
 function MailboxMenu({ mailbox: m, onClose, onCreateChild, onShare }: { mailbox: Mailbox; onClose: () => void; onCreateChild: () => void; onShare: () => void }) {
+  const shared = Object.keys(m.shareWith ?? {}).length > 0;
   const [, navigate] = useLocation();
   const colors = useSettings((s) => s.settings.folderColors);
   const update = useSettings((s) => s.update);
@@ -354,7 +355,13 @@ function MailboxMenu({ mailbox: m, onClose, onCreateChild, onShare }: { mailbox:
       <MenuItem icon={<FolderPlus size={16} />} label="New subfolder" onClick={onCreateChild} disabled={!m.myRights.mayCreateChild} />
       <MenuItem icon={<Pencil size={16} />} label="Rename" onClick={() => void rename()} disabled={isSpecial || !m.myRights.mayRename} />
       <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} />
+      {/* Sharing a mail folder is withdrawn, not removed: Stalwart accepts and
+          stores the share, and it never reaches the other account -- its own
+          docs list calendars, address books and files as shareable and not mail
+          folders. Offering it produced shares that looked real and did nothing.
+          One that already exists can still be cleared here, which is the only
+          reason this entry survives at all. */}
+      {shared && <MenuItem icon={<Share2 size={16} />} label="Stop sharing" onClick={onShare} />}
       <MenuSep />
       <MenuTitle><span className="row gap-4"><Palette size={12} /> Colour</span></MenuTitle>
       <div className="color-grid" style={{ gridTemplateColumns: "repeat(6, 26px)", padding: "4px 10px 8px" }}>