fix(logout): clear the call-volume list and settings-sync stamp (#347)

Logout left lotus-per-user-volume (Element Call's per-person volume map:
whom the previous user called and turned up/down; same origin on the web)
and settings-sync-meta:<userId> (a key naming the account) in
localStorage. clearPlaintextCaches now removes both; device preferences
(`settings`) are kept as before.

Verified: after joining/leaving a call and logging out via Settings, both
keys are gone (unit test added).

Fixes #347

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

src/app/state/plaintextCaches.test.ts

@@ -83,3 +83,14 @@ test('clearPlaintextCaches wipes drafts (#41) and the status message (#204) but
   assert.ok(!removed.includes('cinny_session'), 'session key not this module’s concern');
   assert.ok(store.has('cinny_session'));
 });
+
+test('clearPlaintextCaches removes the call-volume list and the settings-sync stamp (#347)', () => {
+  store.clear();
+  store.set('lotus-per-user-volume', '{"@bob:server":0.3}');
+  store.set('settings-sync-meta:@me:server', '{"lastSyncedAt":1}');
+  store.set('settings', '{}');
+  clearPlaintextCaches('@me:server');
+  assert.equal(store.has('lotus-per-user-volume'), false);
+  assert.equal(store.has('settings-sync-meta:@me:server'), false);
+  assert.equal(store.has('settings'), true, 'device preferences are kept');
+});