Fix four bugs in TciServer causing TCI audio decode/encode failures:

1. buildAudioFrame(): set hdr.length to per-channel sampleCount instead
   of totalFloats (frames × channels). TCI spec requires per-channel count;
   the doubled value caused WSJT-X to overrun into opposite channel data.

2. onRxAudioReady() int16 stereo path: same fix — use audioFrames (per-
   channel) instead of srcSamples (total) for hdr.length.

3. onBinaryMessage() TX path: detect mono vs stereo from payload size
   relative to hdr.length. WSJT-X sends mono float32; treating it as
   stereo halved the effective sample rate, shifting FT8 tone frequencies.

4. audio_stream_sample_type parsing: handle string format names
   ("float32", "int16") in addition to numeric values, since toInt()
   silently returns 0 for non-numeric strings.

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