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>

README.md

@@ -9,7 +9,6 @@ this covers the same settings with a local web page and a command-line tool.
 - Vocal enhancement
 - Hear yourself (headphone monitoring)
 - Mute, kept in sync with the mic's touch pad
-- RGB ring brightness
 - Factory reset
 
 Pure Python, no dependencies. Nothing leaves your machine: the page is served from your own
@@ -80,7 +79,7 @@ keeps its current settings.
 dgm20-web --open             # open the control page (http://localhost:8720)
 dgm20ctl                     # show current settings
 dgm20ctl set nc on nc-level 80
-dgm20ctl set monitor off enhance on brightness 40
+dgm20ctl set monitor off enhance on
 dgm20ctl watch               # print changes made on the mic itself
 dgm20ctl info                # model and firmware
 dgm20ctl factory-reset --yes
@@ -93,7 +92,6 @@ dgm20ctl factory-reset --yes
 | `nc-level` | 0-100: noise cancellation strength |
 | `enhance` | on, off: vocal enhancement |
 | `monitor` | on, off: hear yourself in headphones plugged into the mic |
-| `brightness` | 0-100: RGB ring |
 
 Settings are stored on the mic, so they stay the same after a replug and on other computers.
 
@@ -124,6 +122,11 @@ model that isn't supported yet (see above).
 **`dgm20-web: cannot listen on 127.0.0.1:8720`**: it's already running, for example as the
 service. Open http://localhost:8720, or pick another port with `--port`.
 
+**"the mic is connected but has stopped answering"**: the mic's settings channel froze. This can
+happen if you hold or tap the mic's buttons while settings are being changed from the computer.
+Unplug the mic and plug it back in; its settings are kept. A software USB reset is not enough,
+the mic needs to lose power.
+
 **Commands not found after installing**: `~/.local/bin` isn't on your `PATH`. Add
 `export PATH="$HOME/.local/bin:$PATH"` to your shell profile.