feat(polls): "See who voted" — per-answer voter list

The poll card showed vote counts but never who voted, even though
computePollState already parses a sender for every response. Surface it:

- tallyResponses now also returns voters: Map<answerId, senderId[]>, built in the
  same latest-response-per-sender loop as the counts, so voters can never disagree
  with the numbers (voters.get(id).length === counts.get(id)). +5 unit tests.
- PollContent adds a "Show who voted" toggle, shown only when results are visible
  (disclosed live, or undisclosed after end — so a secret ballot stays secret).
  When on, each answer lists its voters' display names (getMemberName), rendered
  as a sibling of the answer button so the radiogroup keyboard model is untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/components/message/content/PollContent.tsx

@@ -4,6 +4,7 @@ import { Box, Chip, color, config, Icon, Icons, Text, toRem } from 'folds';
 import { RelationsEvent } from 'matrix-js-sdk/lib/models/relations';
 import { MatrixEvent, Room, RoomEvent, PollEvent } from 'matrix-js-sdk';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
+import { getMemberName } from '../../../utils/room';
 import {
   ParsedPoll,
   PollResponse,
@@ -16,7 +17,12 @@ import {
   winningAnswerIds,
 } from '../../../utils/poll';
 
-const EMPTY_TALLY: PollTally = { counts: new Map(), myVotes: new Set(), total: 0 };
+const EMPTY_TALLY: PollTally = {
+  counts: new Map(),
+  voters: new Map(),
+  myVotes: new Set(),
+  total: 0,
+};
 
 type PollState = { tally: PollTally; isEnded: boolean };
 
@@ -106,6 +112,7 @@ export function PollContent({
   const [pending, setPending] = useState<Set<string> | null>(null);
   const [confirmEnd, setConfirmEnd] = useState(false);
   const [ending, setEnding] = useState(false);
+  const [showVoters, setShowVoters] = useState(false);
 
   const refresh = useCallback(() => {
     if (!eventId || !parsed) return;
@@ -194,8 +201,11 @@ export function PollContent({
   const canEnd = !!eventId && !isEnded && (senderId === mx.getUserId() || canRedact);
 
   const myVotes = pending ?? tally.myVotes;
-  const { counts, total } = tally;
+  const { counts, total, voters } = tally;
   const winners = isEnded && showResults ? new Set(winningAnswerIds(counts)) : new Set<string>();
+  // Voter identities are shown under the same rule as the counts: disclosed polls
+  // reveal them live, undisclosed polls only once ended.
+  const canShowVoters = showResults && total > 0;
 
   const handleVote = (answerId: string) => {
     if (!roomId || !eventId || !canVote) return;
@@ -320,8 +330,8 @@ export function PollContent({
               ? 0
               : -1;
           return (
+            <React.Fragment key={id}>
             <button
-              key={id}
               type="button"
               data-poll-answer
               data-selected={selected}
@@ -416,9 +426,32 @@ export function PollContent({
                 )}
               </span>
             </button>
+            {showVoters && canShowVoters && (voters.get(id)?.length ?? 0) > 0 && (
+              <Text
+                size="T200"
+                priority="300"
+                style={{ padding: `0 ${config.space.S300} ${config.space.S100}` }}
+              >
+                {(voters.get(id) ?? []).map((s) => getMemberName(room, s)).join(', ')}
+              </Text>
+            )}
+            </React.Fragment>
           );
         })}
       </Box>
+      {canShowVoters && (
+        <Box>
+          <Chip
+            variant={showVoters ? 'Primary' : 'SurfaceVariant'}
+            radii="Pill"
+            aria-pressed={showVoters}
+            onClick={() => setShowVoters((v) => !v)}
+            before={<Icon size="50" src={Icons.User} />}
+          >
+            <Text size="T200">{showVoters ? 'Hide voters' : 'Show who voted'}</Text>
+          </Chip>
+        </Box>
+      )}
       <Box alignItems="Center" justifyContent="SpaceBetween" gap="200">
         <Text size="T200" priority="300" style={{ minWidth: 0 }}>
           <i>{footerNote}</i>