fix(calls): "Channel Full" counts people, not devices (#342)

useVoiceChannelFull compared the number of MatrixRTC memberships (one per
device) with io.lotus.voice_limit, so one person on desktop + phone, or a
crashed tab whose membership hadn't expired, filled two slots and locked
others out, while the server-side guard (which counts distinct users)
would have admitted them. utils/voiceChannelFull.ts (unit-tested) counts
distinct senders; the header/sidebar counter shows people too.

Verified on the local stack: limit 2, @aud_main in the call on two
devices; @aud_bob's prescreen now shows 1/2 with Join enabled (was
"Channel Full (2/2)"), matching the guard's 200.

Fixes #342

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/voiceChannelFull.test.ts

@@ -0,0 +1,28 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { voiceChannelOccupancy } from './voiceChannelFull';
+
+test('voiceChannelOccupancy: one user on two devices fills one slot, not two', () => {
+  assert.deepEqual(voiceChannelOccupancy(['@a:x', '@a:x'], '@b:x', 2), {
+    channelFull: false,
+    current: 1,
+    max: 2,
+  });
+});
+
+test('voiceChannelOccupancy: full when distinct users reach the limit', () => {
+  assert.deepEqual(voiceChannelOccupancy(['@a:x', '@c:x', '@a:x'], '@b:x', 2), {
+    channelFull: true,
+    current: 2,
+    max: 2,
+  });
+});
+
+test('voiceChannelOccupancy: someone already in the call can always join another device', () => {
+  assert.equal(voiceChannelOccupancy(['@a:x', '@b:x'], '@b:x', 2).channelFull, false);
+});
+
+test('voiceChannelOccupancy: no limit (0) is never full; unknown senders are ignored', () => {
+  assert.equal(voiceChannelOccupancy(['@a:x', '@c:x'], '@b:x', 0).channelFull, false);
+  assert.equal(voiceChannelOccupancy([undefined, '@a:x'], '@b:x', 2).current, 1);
+});