Hold GREETING state for at least 4 s so UI always shows recognition

When TTS is unavailable speak() returns in milliseconds, causing the
GREETING state to be entered and exited before the UI's 1-second poll
could catch it — the user saw the robot go straight to sleep with no
feedback at all.

Fix: record the time before speak(); after it returns, sleep the
remaining time up to a 4-second minimum. The UI is guaranteed to see
at least three poll cycles with state="greeting" and the face snapshot,
regardless of whether TTS is working.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

recognizer/main.py

@@ -187,9 +187,13 @@ class Recognizer(ReachyMiniApp):
             elif state == State.GREETING:
                 with _lock:
                     _shared["state"] = "greeting"
-                # Speak first; GUI already shows the name via /status.
-                # If TTS is unavailable the name remains visible in the UI.
+                greeting_t0 = time.time()
                 speak(f"Hi {greeting_name}!", reachy_mini)
+                # Keep greeting visible for at least 4 s so the UI poll
+                # (1 s interval) always catches it, even when TTS is absent.
+                remaining = 4.0 - (time.time() - greeting_t0)
+                if remaining > 0:
+                    time.sleep(remaining)
                 reachy_mini.goto_sleep()
                 with _lock:
                     _shared["recognized_name"] = None