fix: fall back from invalid presence timezone

test/dave-presence.test.mjs

@@ -89,3 +89,11 @@ test('local clock uses the profile timezone', () => {
   assert.deepEqual(localClock(new Date('2026-07-06T12:30:00Z'), 'Europe/Berlin'), { day: '2026-07-06', time: '14:30' });
   assert.equal(localClock(new Date(), 'Invalid/Timezone'), null);
 });
+
+test('invalid profile timezone falls back to configured operational timezone', async () => {
+  const { presence, messages, calls } = setup({ profile: { timezone: 'not-a-timezone' } });
+  const outcome = await presence.tick(new Date('2026-07-06T12:00:00Z'));
+  assert.equal(outcome.reason, 'sent');
+  assert.equal(messages.length, 1);
+  assert.match(calls[0].prompt, /timezone: UTC/);
+});