feat(iios): expose senderId (stable externalId) on MessageDto

The app needs a reliable "is this message mine?" signal. senderActorId worked only
with a lazily-learned actorId, which Supabase token-refresh wipes. senderId is the
sender's externalId (email/username) — always present and comparable to the session
user — so message alignment is deterministic. Verified: senderId round-trips = email.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/src/messaging/message.service.ts

@@ -20,6 +20,7 @@ export interface MessageDto {
   id: string;
   threadId: string;
   senderActorId: string;
+  senderId: string; // the sender's stable externalId (email/username) — reliable "is this mine?"
   senderName: string;
   content: string;
   contentRef?: string;
@@ -506,6 +507,7 @@ export class MessageService {
       id: interaction.id,
       threadId,
       senderActorId: interaction.actorId ?? '',
+      senderId: interaction.actor?.sourceHandle?.externalId ?? '',
       senderName: interaction.actor?.displayName ?? interaction.actor?.sourceHandle?.externalId ?? 'unknown',
       content: text?.bodyText ?? '',
       contentRef: file?.contentRef ?? undefined,