Security says why a mail app won't connect, when legacy protocols are off

When the mail server has turned off legacy mail protocols -- IMAP, POP3,
ManageSieve and sending from mail apps -- for this account, whether for
the whole server or for the account's organization, Settings › Security
says so at the top of App passwords, the section people come to when a
phone won't connect:

  Your organization allows only {app} and JMAP apps, so phone and desktop
  mail apps can't connect to this account.

This is INBUXA's legacy-protocols LP-19. The server reports it per
account as legacyProtocols on the urn:inbuxa:jmap account capability
(contract C-1); anything short of a plain "disabled" -- an older server,
another server, no session yet -- reads as on, so the line never appears
where it isn't true.

The app's name comes from {app}, as everywhere else. Translated into all
nine languages, in the register each catalog already uses.

web/src/jmap/client.ts

@@ -22,6 +22,21 @@ export const CAP = {
 /** Stalwart's own capability, which carries its `x:` registry methods. */
 export const STALWART_CAP = "urn:stalwart:jmap";
 
+/** INBUXA's own capability (contract C-1), on the signed-in account. */
+export const INBUXA_CAP = "urn:inbuxa:jmap";
+
+/**
+ * Whether legacy mail protocols -- IMAP, POP3, ManageSieve, sending from mail
+ * apps -- are off for this account: the stricter of the server's switch and
+ * its organization's, as the server reports it (legacy-protocols LP-19). Any
+ * server that doesn't say reads as on.
+ */
+export function legacyProtocolsOff(session: JmapSession | null, accountId: Id | null): boolean {
+  if (!session || !accountId) return false;
+  const cap = session.accounts[accountId]?.accountCapabilities?.[INBUXA_CAP] as { legacyProtocols?: string } | undefined;
+  return cap?.legacyProtocols === "disabled";
+}
+
 export class JmapMethodError extends Error {
   constructor(
     public readonly method: string,