fix(presence): leaving Do Not Disturb restores your status instead of "dnd" (#283)

Lotus encodes DND as status_msg "dnd". After switching back to Auto, the
heartbeat read the server's status, adopted "dnd" as the user's own and
kept re-sending it, so the custom status was lost and other clients kept
showing "dnd"; the Settings status field showed "dnd" too.

pickHeartbeatStatus (unit-tested) never adopts the "dnd" sentinel and
falls back to the device's real status; Profile neither mirrors it into
the local status nor shows it in the field.

Verified on the local Synapse: "🎮 Gaming" → DND (unavailable/dnd) → Auto
→ online/"🎮 Gaming" (before: online/"dnd").

Still open in #283: while in DND other clients show the literal "dnd";
that needs MSC3026 busy presence on the homeserver (matrix#19).

Refs #283

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

src/app/utils/ownStatusSync.test.ts

@@ -48,3 +48,34 @@ test("a fresh local save wins over a server read that hasn't caught up", () => {
     'previous',
   );
 });
+
+test('pickHeartbeatStatus: the "dnd" sentinel is never adopted as the user status (#283)', () => {
+  const base = { lastLocalWriteAt: 0, now: 100_000 };
+  // leaving DND: the server still says "dnd" → restore the real local status
+  assert.equal(
+    pickHeartbeatStatus({
+      ...base,
+      local: '🎮 Gaming',
+      server: { presence: 'unavailable', status_msg: 'dnd' },
+    }),
+    '🎮 Gaming',
+  );
+  // a local copy that had absorbed the sentinel becomes empty, not "dnd"
+  assert.equal(
+    pickHeartbeatStatus({
+      ...base,
+      local: 'dnd',
+      server: { presence: 'online', status_msg: 'dnd' },
+    }),
+    '',
+  );
+  // ordinary statuses are unaffected
+  assert.equal(
+    pickHeartbeatStatus({
+      ...base,
+      local: 'old',
+      server: { presence: 'online', status_msg: 'new' },
+    }),
+    'new',
+  );
+});