Add unread message indicators for rooms (#38)

Shows a dot on rooms with unread messages in the sidebar, updated live
over WebSocket. Reuses the same offline-member audience computation
already used for push notifications: a member gets the real-time signal
whenever they aren't currently connected to that room's channel, which
correctly covers both "room not open" and "room open but tab
backgrounded" (the client leaves a room's channel while hidden).

Persisted server-side via a new room_memberships.last_read_at column so
state survives reload and stays consistent across devices, advanced by
an explicit mark-read call the frontend makes on room-open and on each
live message received while the room is genuinely visible -- gated on a
live visibility check, not a cached ref, so a backgrounded-but-open room
keeps accumulating unread instead of auto-marking-read the instant a
message arrives somewhere it can't be seen.

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

frontend/src/components/RoomRow.tsx

@@ -32,6 +32,7 @@ export function RoomRow({ room, colorIndex, active }: RoomRowProps) {
         </div>
         {room.description && <div className="room-row-subtitle">{room.description}</div>}
       </div>
+      {room.has_unread && !active && <span className="room-row-unread-dot" aria-label="Unread messages" />}
     </Link>
   )
 }