Show an assigned shared mailbox as shared, not as a locked account

The server now marks a shared mailbox (support@, legal@) as a
delegation of kind "sharedMailbox" (multi-account spec, MA-S). Without
this, the webmail would show one exactly as it shows a locked account:
a red bar, a padlock in the tab and on the brand, and its calendars and
files in place of the reader's own.

Now such a mailbox is listed with the other shared mailboxes under
"Mail to show", opens with the shared bar, which also gives the
person's access level and whether they can send as it, and changes
mail only. Its access level still applies exactly as a lock's does:
read changes nothing, organize never deletes, no sending without
send-as. Found without asking Mailbox/get, since the server's mark says
it is mail.

A server that sends no kind is treated as before: every delegation is a
lock. No new strings. typecheck and vitest (174 files, 1534 tests)
pass.

web/src/store/session.ts

@@ -188,8 +188,8 @@ export const useSession = create<SessionState>((set, get) => ({
   viewAccountFor(cap) {
     const { session, viewing } = get();
     const viewed = viewing ? session?.accounts[viewing] : undefined;
-    // A shared or group mailbox in view is mail only (MA-A)
-    if (viewing && viewed && delegationOf(session, viewing) && cap in (viewed.accountCapabilities ?? {})) return viewing;
+    // A shared or group mailbox in view is mail only (MA-A, MA-S)
+    if (viewing && viewed && delegationOf(session, viewing)?.kind === "lock" && cap in (viewed.accountCapabilities ?? {})) return viewing;
     return ownAccountForCapability(session, cap);
   },
 }));