fix(a11y): add role=log aria-live to message timeline (P3-4)

Screen readers now announce new messages politely via role='log' +
aria-live='polite' on the message container in RoomTimeline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/room/RoomTimeline.tsx

@@ -2102,6 +2102,9 @@ export function RoomTimeline({ room, eventId, roomInputRef, editor }: RoomTimeli
           <Box
             direction="Column"
             justifyContent="End"
+            role="log"
+            aria-label="Message timeline"
+            aria-live="polite"
             style={{ minHeight: '100%', padding: `${config.space.S600} 0` }}
           >
             {!canPaginateBack && rangeAtStart && getItems().length > 0 && (