Mail list: name everyone who wrote in the thread, and call the user "me"

The sender column showed participants[0], and participants are rebuilt
newest message first — so replying to a thread put the user in that slot
and the list named them instead of the person who wrote in.

Thread summaries (list and search) now carry `senders`: the distinct From
addresses of the thread's visible messages, oldest first, with the same
Trash/Spam visibility as the message count. The list joins them and shows
the user's own account addresses as "me" ("me, Alice" for a thread the
user started), and the avatar prefers the other side of the conversation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/api/search.py

@@ -10,7 +10,7 @@ from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.api.schemas import SearchOut, SearchThreadHit
-from app.api.threads import _summary
+from app.api.threads import _summary, thread_senders
 from app.db import get_session
 from app.identity.deps import CurrentUser
 from app.models import Thread
@@ -56,8 +56,12 @@ async def search(
             select(Thread).where(Thread.id.in_(order), Thread.user_id == user.id)
         )
     }
+    senders = await thread_senders(db, user.id, list(threads))
     results = [
-        SearchThreadHit(thread=_summary(threads[tid]), highlights=highlights.get(tid, {}))
+        SearchThreadHit(
+            thread=_summary(threads[tid], senders=senders.get(tid)),
+            highlights=highlights.get(tid, {}),
+        )
         for tid in order
         if tid in threads
     ]