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/utils/poll.test.ts

@@ -70,6 +70,51 @@ test('tallyResponses: re-voting after an empty clear counts the newest non-empty
   assert.equal(total, 1);
 });
 
+test('tallyResponses: voters list the senders per answer, consistent with counts', () => {
+  const responses: PollResponse[] = [
+    { sender: '@a:s', ts: 1, answerIds: ['1'] },
+    { sender: '@b:s', ts: 1, answerIds: ['1'] },
+    { sender: '@c:s', ts: 1, answerIds: ['2'] },
+  ];
+  const { counts, voters } = tallyResponses(responses, me);
+  assert.deepEqual([...(voters.get('1') ?? [])].sort(), ['@a:s', '@b:s']);
+  assert.deepEqual(voters.get('2'), ['@c:s']);
+  // voters must always agree with counts
+  for (const [id, list] of voters) {
+    assert.equal(list.length, counts.get(id));
+  }
+});
+
+test('tallyResponses: a re-vote moves the sender to the new answer in voters', () => {
+  const responses: PollResponse[] = [
+    { sender: '@a:s', ts: 1, answerIds: ['1'] },
+    { sender: '@a:s', ts: 5, answerIds: ['2'] }, // changed vote
+  ];
+  const { voters } = tallyResponses(responses, me);
+  assert.equal(voters.get('1'), undefined); // no longer under the old answer
+  assert.deepEqual(voters.get('2'), ['@a:s']);
+});
+
+test('tallyResponses: a cleared latest response drops the sender from every voter list', () => {
+  const responses: PollResponse[] = [
+    { sender: '@a:s', ts: 1, answerIds: ['1', '2'] },
+    { sender: '@a:s', ts: 2, answerIds: [] }, // cleared
+  ];
+  const { voters } = tallyResponses(responses, me);
+  assert.equal(voters.get('1'), undefined);
+  assert.equal(voters.get('2'), undefined);
+});
+
+test('tallyResponses: multi-select lists a sender under each chosen answer', () => {
+  const responses: PollResponse[] = [
+    { sender: '@a:s', ts: 1, answerIds: ['1', '3'] },
+    { sender: '@b:s', ts: 1, answerIds: ['3'] },
+  ];
+  const { voters } = tallyResponses(responses, me);
+  assert.deepEqual(voters.get('1'), ['@a:s']);
+  assert.deepEqual([...(voters.get('3') ?? [])].sort(), ['@a:s', '@b:s']);
+});
+
 test('resultsVisible: disclosed always, undisclosed only when ended', () => {
   assert.equal(resultsVisible(false, false), true); // disclosed, open
   assert.equal(resultsVisible(false, true), true); // disclosed, ended