Also email room subscribers on the first unread message, not just mentions (#67)

The previous pass only emailed on a mention. Corrected scope: the
room's first unread message triggers one debounced email (same shape
as #66), and every mention additionally emails regardless of that
debounce, since a mention shouldn't get silently absorbed by an
earlier plain message's already-sent notification.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/api/rooms.ts

@@ -68,9 +68,10 @@ export function hideDm(roomId: string): Promise<void> {
   return apiFetch<void>(`/api/rooms/${roomId}/hide`, { method: 'POST' })
 }
 
-// #67: per-viewer opt-in for email-on-mention in this room. 400s for a DM
-// (backend rejects it -- see set_room_email_notifications), so callers
-// should only expose the toggle for a non-DM room.
+// #67: per-viewer opt-in for email on this room's first unread message
+// and every mention. 400s for a DM (backend rejects it -- see
+// set_room_email_notifications), so callers should only expose the
+// toggle for a non-DM room.
 export function updateRoomNotifications(roomId: string, emailNotifications: boolean): Promise<void> {
   return apiFetch<void>(`/api/rooms/${roomId}/notifications`, {
     method: 'PATCH',