Drop the brightness control, survive and explain a frozen mic

- Remove RGB brightness: firmware 2.1.2 stores the value but the LEDs
  never change, so the control did nothing. PROTOCOL.md notes why.
- Lock the device per transaction (flock) so the CLI and the web
  service can't interleave and steal each other's replies.
- Detect a mic that is plugged in but no longer answering and tell the
  user to replug it (a USB reset is not enough; settings survive).
  Seen when hardware buttons are held while settings are being written.
- Web page keeps the last known value when a poll comes back partial,
  instead of drawing unanswered settings as off.
- New screenshot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_protocol.py

@@ -60,5 +60,21 @@ class Encoding(unittest.TestCase):
             encode("volume", 3)
 
 
+
+class NotRespondingDetection(unittest.TestCase):
+    def test_all_unanswered_reads_raise(self):
+        from dgm20.device import Mic, NotResponding
+        mic = Mic.__new__(Mic)          # no device needed: reads always time out
+        mic.read = lambda addr: None
+        with self.assertRaises(NotResponding):
+            mic.state()
+
+    def test_partial_answers_are_fine(self):
+        from dgm20.device import Mic
+        mic = Mic.__new__(Mic)
+        mic.read = lambda addr: 1 if addr == 0x2022 else None
+        self.assertTrue(mic.state()["mute"])
+
+
 if __name__ == "__main__":
     unittest.main()