Splits the legacy single "CwDecodeOverlay" decode flag into two
independent toggles — RX (received signal, existing behaviour) and TX
(operator's own keying, fed from the client-side sidetone) — wrapped
in a nested AppSettings["CwDecoder"] JSON blob so the feature owns one
root key rather than two new flat ones.  Legacy flat key is migrated
on first read so existing users keep their RX-on default.

TX path taps the CwSidetoneGenerator's per-block mono signal via a
SampleTap callback set once at AudioEngine ctor.  AudioEngine
downsamples 48→24 kHz, duplicates to stereo float32 (matching the
CwDecoder feedAudio shape), and emits txDecodeAudioReady when the
runtime gate is on.  MainWindow flips that gate from moxChanged and
toggle changes so audio only flows during transmit.

A dedicated CwDecoder instance handles TX so decoded text lands in
PanadapterApplet::appendCwTextTx and renders with a "[TX]" prefix and
distinct cyan color, distinguishable from received CW in the same
panel.  RX/TX toggles in RadioSetupDialog replace the single "Decode"
button.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Follow-up fixes after live-testing the TX-side CW decoder feature.

- **CwxLocalKeyer word gap**: produced 4u of silence at every CWX word
  boundary instead of the standard 7u — `firstChar = true` after a
  space suppressed the next letter's CharGap.  4u fell inside ggmorse's
  3u inter-character classification window, erasing word boundaries in
  the TX decode panel and also diverging from standard CW timing for
  the local sidetone listener.  Remove the firstChar reset so the next
  letter contributes its 3u CharGap, giving WordGap(4u) + CharGap(3u)
  = 7u total.

- **TX-tap MOX gate**: AudioEngine's CW-decode-TX tap was AND'd with
  `isMox() || isTransmitting()`, but CLAUDE.md is explicit that the
  radio never sends `mox=` in status, and CW keying goes through netcw
  rather than the conventional MOX interlock.  The result was a tap
  that never fired on actual CW transmissions.  Gate solely on the
  `CwDecodeSettings::txEnabled()` toggle; the sidetone generator
  already self-gates on key state so silence buffers between bursts
  cost nothing.

- **CwDecoder::setKnownParameters**: new API on CwDecoder that locks
  both pitch and speed to externally-supplied values (with widened
  pitch range) and pushes ggmorse params live.  Used by MainWindow to
  feed the TX decoder the operator's known cw_pitch / cw_speed from
  the P/CW applet, so ggmorse classifies gap durations against the
  correct unit length and word boundaries become " " separators.  Hook
  TransmitModel::phoneStateChanged into refreshCwDecodeState so the
  values track slider movement live.

- **Renderer cleanup**: drop the per-emit "[TX] " prefix that produced
  unreadable "[TX] E [TX] M [TX] T …" output.  Cyan color alone is the
  visual signal; a single space separator is inserted on Rx↔Tx
  transitions so the two colored runs don't merge.

Tested live on FLEX-8600 with CWX-driven keying: `CQ CQ CQ DE KK7GWY K`
decodes correctly with proper word spacing on the panel.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>