fix(email): don't probe IMAP for send-only (SMTP-only) accounts (#4830)

An account configured with SMTP only (no imap_host) has no inbox, but the
inbox list path still called _imap_connect, which handed an empty host to
imaplib. imaplib.IMAP4("", 993) silently dials localhost:993 and fails with
"[Errno 111] Connection refused", so the email panel's poll logged a
"Failed to list emails" ERROR every ~60s and surfaced a scary error in the UI.

_imap_connect now fails fast with a typed EmailNotConfiguredError (subclass of
RuntimeError, so existing broad handlers keep working) when no imap_host is set,
and the inbox list returns an empty result for that case instead of an error.
SMTP send is unaffected.

routes/email_helpers.py

@@ -40,6 +40,16 @@ from src.secret_storage import decrypt as _decrypt
 logger = logging.getLogger(__name__)
 
 
+class EmailNotConfiguredError(RuntimeError):
+    """Raised when an IMAP operation is attempted on an account that has no
+    inbox configured (e.g. a send-only / SMTP-only account).
+
+    Subclasses RuntimeError so existing broad ``except Exception`` handlers
+    keep working; callers that want to treat "no inbox" as an empty result
+    rather than a failure can catch this type specifically.
+    """
+
+
 def _xoauth2_raw(user: str, access_token: str) -> str:
     """The SASL XOAUTH2 initial-response string (unencoded).
 
@@ -929,6 +939,14 @@ def _imap_connect(account_id: str | None = None, owner: str = "",
     # `timeout` is overridable so short-lived callers (e.g. the service-health
     # probe) can impose a tighter budget than the default IMAP timeout.
     cfg = _get_email_config(account_id, owner=owner)
+    # Send-only (SMTP-only) account: no IMAP host means there is no inbox to
+    # read. Bail out with a clear, typed error instead of handing an empty
+    # host to imaplib — IMAP4("", 993) silently dials localhost:993 and fails
+    # with a confusing "[Errno 111] Connection refused" on every inbox poll.
+    if not cfg.get("imap_host"):
+        raise EmailNotConfiguredError(
+            f"IMAP is not configured for account {cfg.get('account_name') or 'default'!r}"
+        )
     # Connection mode:
     #   STARTTLS on → plain + upgrade
     #   STARTTLS off + port 993 → implicit SSL (IMAPS)