fix(jmap): request /jmap/session directly to survive iOS auth-drop on redirect

Signing in from the iOS build against a Stalwart server fails with "No
account found in JMAP session".

Stalwart's discovery endpoint /.well-known/jmap answers with a 307 to
/jmap/session. iOS NSURLSession strips the Authorization header when it
auto-follows that redirect, so the app gets an unauthenticated session
back — HTTP 200, empty accounts — and resolveAccountId throws. Browsers
keep the auth header on same-origin redirects, which is why the webmail
never saw this; it only bites the native app on iOS.

Request /jmap/session directly so the header stays attached, falling
back to /.well-known/jmap on 404 for servers that don't expose the
Stalwart path.

Also treat a 200 with no accounts as an auth failure. Stalwart answers
that way for missing or invalid credentials instead of sending a 401, so
a user who fat-fingers their password currently gets "No account found
in JMAP session" rather than "Invalid credentials".

Tests updated for both behaviours; full suite passes (the pre-existing
auth-store.test.ts parse failure on main is unrelated).

src/api/__tests__/jmap-client.test.ts

@@ -90,7 +90,7 @@ describe('JMAPClient', () => {
       await client.connect('https://mail.example.com///', 'user', 'pass');
 
       expect(global.fetch).toHaveBeenCalledWith(
-        'https://mail.example.com/.well-known/jmap',
+        'https://mail.example.com/jmap/session',
         expect.any(Object),
       );
     });
@@ -281,7 +281,9 @@ describe('JMAPClient', () => {
       expect(client.accountId).toBe('only-acc');
     });
 
-    it('should throw if no accounts at all', async () => {
+    it('should throw on an empty (unauthenticated) session', async () => {
+      // Stalwart returns 200 with empty accounts for invalid credentials;
+      // the client surfaces that as an auth failure.
       const session = {
         ...MOCK_SESSION,
         primaryAccounts: {},
@@ -290,7 +292,7 @@ describe('JMAPClient', () => {
       global.fetch = mockFetch([{ status: 200, json: session }]) as any;
 
       await expect(client.connect('https://mail.example.com', 'user', 'pass'))
-        .rejects.toThrow('No account found');
+        .rejects.toThrow('Invalid credentials');
     });
   });
 });

src/api/jmap-client.ts

@@ -361,18 +361,30 @@ export class JMAPClient {
   // ── Session Discovery ─────────────────────────────────
 
   private async fetchSession(baseUrl: string): Promise<JMAPSession> {
-    const url = `${baseUrl}/.well-known/jmap`;
     await this.ensureFreshToken();
-    const doFetch = () =>
+    // Stalwart's discovery endpoint /.well-known/jmap 307-redirects to
+    // /jmap/session. On iOS, NSURLSession drops the Authorization header when it
+    // auto-follows that redirect, so the app receives an unauthenticated,
+    // empty-accounts session and fails with "No account found in JMAP session".
+    // Request the session endpoint directly so the auth header stays attached;
+    // fall back to the standard well-known path for non-Stalwart servers.
+    const doFetch = (url: string) =>
       secureFetch(url, {
         headers: {
           Authorization: this.authHeader,
           Accept: 'application/json',
         },
       });
-    let response = await doFetch();
+    const primary = `${baseUrl}/jmap/session`;
+    const fallback = `${baseUrl}/.well-known/jmap`;
+    const run = async () => {
+      const r = await doFetch(primary);
+      return r.status === 404 ? doFetch(fallback) : r;
+    };
+
+    let response = await run();
     if (response.status === 401 && (await this.forceRefreshToken())) {
-      response = await doFetch();
+      response = await run();
     }
 
     if (response.status === 401) {
@@ -382,7 +394,17 @@ export class JMAPClient {
       throw new Error(`Session discovery failed: ${response.status} ${response.statusText}`);
     }
 
-    return response.json();
+    const session: JMAPSession = await response.json();
+    // Stalwart returns HTTP 200 with empty accounts for missing/invalid
+    // credentials (rather than 401). Treat an empty session as an auth failure
+    // so the user sees "Invalid credentials" instead of "No account found".
+    const hasAccount =
+      Object.keys(session.primaryAccounts ?? {}).length > 0 ||
+      Object.keys(session.accounts ?? {}).length > 0;
+    if (!hasAccount) {
+      throw new AuthenticationError('Invalid credentials');
+    }
+    return session;
   }
 
   private resolveAccountId(session: JMAPSession): string {