Revert "Mail list: show who wrote, not everyone involved"

This reverts commit 550b8cf503.

backend/app/threading_/threader.py

@@ -197,16 +197,6 @@ async def denormalize_thread(session: AsyncSession, thread_id: uuid.UUID) -> Non
                 seen_emails.add(email)
                 participants.append({"name": addr.get("name", ""), "email": addr.get("email", "")})
 
-    senders: list[dict[str, str]] = []
-    seen_senders: set[str] = set()
-    for msg in messages:  # newest first => most recent writer leads
-        for addr in msg.from_addr:
-            email = (addr.get("email") or "").lower()
-            if email and email not in seen_senders:
-                seen_senders.add(email)
-                senders.append({"name": addr.get("name", ""), "email": addr.get("email", "")})
-
-    thread.senders = senders
     thread.message_count = len(messages)
     thread.unread_count = sum(1 for m in messages if "\\Seen" not in m.flags)
     thread.has_attachments = any(m.has_attachments for m in messages)