fix(email): guard against IndexError when IMAP search returns empty list

imap.uid('search') can return data=[] when the mailbox is empty or
has no matching messages. Accessing data[0] without checking len first
raises IndexError: list index out of range.

Fixed at both call sites in gateway/platforms/email.py:
- Line 233 (connect): ALL search on startup
- Line 298 (fetch): UNSEEN search in the polling loop

Closes #2137

gateway/platforms/email.py

@@ -230,7 +230,7 @@ class EmailAdapter(BasePlatformAdapter):
             # Mark all existing messages as seen so we only process new ones
             imap.select("INBOX")
             status, data = imap.uid("search", None, "ALL")
-            if status == "OK" and data[0]:
+            if status == "OK" and data and data[0]:
                 for uid in data[0].split():
                     self._seen_uids.add(uid)
             imap.logout()
@@ -295,7 +295,7 @@ class EmailAdapter(BasePlatformAdapter):
             imap.select("INBOX")
 
             status, data = imap.uid("search", None, "UNSEEN")
-            if status != "OK" or not data[0]:
+            if status != "OK" or not data or not data[0]:
                 imap.logout()
                 return results