Add fast clear animation

The old clear mechanism would write an empty cell in every layer,
which would take (0.3 seconds) * (11 layers) == about 3 seconds.

The new mechanism drives an animation which overwrites every character
slot simultaneously, taking only 0.1 seconds. A nice ~30x speedup.

* Fix the transcription exponential backoff logic. Saying new things
  will reset the delay to the minimum again.
* Clearing the board will also reset the transcription delay back to
  the minimum.
* Tune the noise detection minimum to 0.2 instead of 0.1. Speaking
  softly into the mic seems to fail to exceed the 0.1 threshold pretty
  often.

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