Mail list: show the sender only, keep the participant count

The first column listed every participant, so an ordinary email read
"Sender, You". It now shows the sender — `participants[0]`, which is the newest
message's from address because the threader adds each message's sender ahead of
its recipients.

The +N badge beside the avatar stays: the name is narrowed, the "how many people
are in this" signal is not.

Nine lines in one component. An earlier attempt at this added a denormalized
`threads.senders` column and a backfill migration so that a thread with several
writers could list them all; that was more machinery than the request warranted
and is reverted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018SqJYt7jD7oUEkoY68sBZo

frontend/src/components/list/MessageList.tsx

@@ -119,9 +119,13 @@ export function MessageList({
           const selected = selectedId === thread.id;
           const checked = checkedIds.has(thread.id);
           const dimmed = dimmedIds?.has(thread.id) ?? false;
+          // `participants` is from + to + cc, newest message first — so the first
+          // entry is the sender and the rest are recipients (usually just you).
+          // The list shows the sender only.
           const primary = thread.participants[0];
-          const names =
-            thread.participants.map((p) => p.name || p.email).join(",  ") || "(no one)";
+          const names = primary ? primary.name || primary.email : "(no one)";
+          // The badge still counts everyone involved — the name is narrowed, the
+          // "how many people are in this" signal is not.
           const extra = Math.max(0, thread.participants.length - 1);
           const avatarBg = `hsl(${hue(primary?.email || primary?.name || "?")} 45% 45%)`;
           const stop = (fn: () => void) => (e: React.MouseEvent) => {