feat(gateway): butler filler line while Tatlock works

Tatlock turns take 10-25s, which is a long silence after a request. Speak
a canned "Let me check on that for you, sir" immediately (synthesized once
and cached), then re-assert the thinking state so the device keeps its
effort-face spinner up until the real reply arrives. On the device, guard
the playback-done handler so the filler audio finishing doesn't drop the
spinner back to idle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LKPbR6DY2JygHbyLjxm7Uu

firmware/main/desklock_main.c

@@ -122,7 +122,12 @@ static void app_task(void *arg)
 
 void app_on_playback_done(void)
 {
-    face_set(FACE_IDLE);
+    /* Only fall back to idle if we're still the speaker. After the butler filler
+     * line, the gateway re-asserts "thinking" (FACE_EFFORT) while Tatlock works —
+     * don't clobber that spinner when the filler audio finishes. */
+    if (face_get() == FACE_SPEAKING) {
+        face_set(FACE_IDLE);
+    }
 }
 
 #if FACE_LOADTEST