No address step before signing in when there's only one mail server

The server's own page asks for the username, so with a single server the
sign-in page keeps only the own-device choice. With several servers the
address still comes first, since its domain picks the server. One new string,
in all nine catalogues.

server/src/app.ts

@@ -41,7 +41,7 @@ import {
   revokeAppPassword,
 } from "./account.js";
 import { imageProxyHandler } from "./imageproxy.js";
-import { SignInError, finish as finishSignIn, needsRefresh, oauthEnabled, refreshTokens, start as startSignIn, type TokenSet } from "./oauth.js";
+import { SignInError, finish as finishSignIn, needsRefresh, oauthEnabled, refreshTokens, singleServer, start as startSignIn, type TokenSet } from "./oauth.js";
 import { icsProxyHandler } from "./icsproxy.js";
 import { staticHandler } from "./static.js";
 
@@ -384,6 +384,9 @@ export function createApp(basePath = config.basePath): Hono<Env> {
       settingsPolicy: config.settingsPolicy,
       /* "oauth": sign in on the mail server's own page (see oauth.ts). */
       signIn: oauthEnabled() ? "oauth" : "password",
+      /* With "oauth": true when the server's page can take it from here, so
+         the sign-in form doesn't ask for an address first. */
+      signInDirect: oauthEnabled() && singleServer(),
     }),
   );
 

server/src/oauth.test.ts

@@ -84,6 +84,18 @@ after(() => {
 test("the configuration tells the web app to use the server's page", async () => {
   const body = await jsonOf(await call("/api/config"));
   assert.equal(body.signIn, "oauth");
+  assert.equal(body.signInDirect, true, "one mail server: its page asks for the username, not ihasmail");
+});
+
+test("with one mail server, sign-in starts without an address", async () => {
+  const res = await call("/api/auth/oauth/start");
+  assert.equal(res.status, 302);
+  const to = new URL(res.headers.get("location")!);
+  assert.equal(to.searchParams.has("login_hint"), false);
+  const approved = await fetch(to, { redirect: "manual" });
+  const back = new URL(approved.headers.get("location")!);
+  assert.equal((await call(`/api/auth/callback${back.search}`)).headers.get("location"), "/");
+  assert.equal((await call("/api/auth/session")).status, 200);
 });
 
 test("the password form is refused: ihasmail never sees a password", async () => {

server/src/oauth.ts

@@ -52,6 +52,16 @@ export function oauthEnabled(): boolean {
   return Boolean(config.oauthClientSecret);
 }
 
+/**
+ * Whether every account is on the same server. Then sign-in needs no address
+ * first: the server's page asks for the username itself. With several servers
+ * (STALWART_SERVERS_FILE), the domain picks the server, so the address comes
+ * first.
+ */
+export function singleServer(): boolean {
+  return Object.values(config.stalwartServers).every((url) => url === config.stalwartUrl);
+}
+
 /** The one redirect URI registered for this client on the server. */
 export function redirectUri(): string {
   return `${config.publicUrl}${config.basePath}/api/auth/callback`;