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>

frontend/src/i18n/en.ts

@@ -67,6 +67,7 @@ export const en = {
   "nav.refresh": "Refresh all folders",
 
   "mail.noConversations": "No conversations.",
+  "mail.me": "me",
   "mail.selectConversation": "Select a conversation.",
   "mail.loadingThread": "Loading thread…",
   "mail.expandAll": "Expand all",