Mail list: show who wrote, not everyone involved

The first column listed a thread's `participants`, which the threader builds from
from + to + cc — so every ordinary email read "Sender, You", and the +N badge
beside the avatar counted the user themselves.

`participants` cannot simply be narrowed: the subject-fallback join matches an
incoming message's recipients against it (threader._by_subject), so dropping
recipients would change how mail is threaded. Hence a separate `threads.senders`,
holding only from addresses, newest writer first — which is also the right thing
for a conversation where several people replied.

Migration 0019 backfills it, so existing threads do not wait to be touched again.
The list falls back to `participants` if a row somehow has no senders, rather
than rendering an empty column.

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

frontend/src/components/list/MessageList.tsx

@@ -119,10 +119,12 @@ export function MessageList({
           const selected = selectedId === thread.id;
           const checked = checkedIds.has(thread.id);
           const dimmed = dimmedIds?.has(thread.id) ?? false;
-          const primary = thread.participants[0];
-          const names =
-            thread.participants.map((p) => p.name || p.email).join(",  ") || "(no one)";
-          const extra = Math.max(0, thread.participants.length - 1);
+          // Senders, not participants: the latter includes recipients, so every
+          // ordinary email read "Sender, You" and the +N badge counted the user.
+          const people = thread.senders.length ? thread.senders : thread.participants;
+          const primary = people[0];
+          const names = people.map((p) => p.name || p.email).join(",  ") || "(no one)";
+          const extra = Math.max(0, people.length - 1);
           const avatarBg = `hsl(${hue(primary?.email || primary?.name || "?")} 45% 45%)`;
           const stop = (fn: () => void) => (e: React.MouseEvent) => {
             e.stopPropagation();

frontend/src/lib/types.ts

@@ -31,6 +31,9 @@ export interface ThreadSummary {
   id: string;
   subject: string;
   snippet: string;
+  /** Who wrote in the thread, newest first. The list shows these. */
+  senders: Address[];
+  /** Everyone involved, recipients included — used for threading, not display. */
   participants: Address[];
   account_ids: string[];
   message_count: number;