fix: web spectrum tuning range from backend caps; remove dead code

Fix: dragging/wheeling the spectrum in the web UI clamped frequency to a
hardcoded HF ceiling (vfoMax=60 MHz), so on Pluto VHF/UHF it snapped down
to 60 MHz and couldn't pan up (only the VFO wheel worked, via a different
limit). Push the backend's tuning range (BackendCaps.MinFreqHz/MaxFreqHz)
to the web alongside the band plan; JS vfoMin/vfoMax use freq_min/freq_max
from state. The transverter branch (XVTR_CUR>=0) is unchanged — it keeps
the wide-open range since the displayed RF freq is translated to IF and
clamped server-side.

Dead-code cleanup (audit of this session's work):
- SampleRateMode/TSampleRateMode: orphaned when the sample-rate
  unification replaced the srmContinuous branch in ApplyBackendCapsToUI;
  no readers left. Removed (+ now-unused PLUTO_MAX_SR).
- Pre-existing write-only/unused: TBackendCaps.MaxSampleRate,
  SampleRateOverlay.SPAN_NAMES, MainForm.BAND_FREQ.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

HPSDRNetwork.pas

@@ -631,8 +631,6 @@ begin
   Result.HasRFBandwidth  := False;
   Result.HasFullDuplex   := True;
   Result.MinSampleRate   := 48000;
-  Result.MaxSampleRate   := 1536000;
-  Result.SampleRateMode  := srmDiscrete;
   // Бэкенд — единственный источник своих рейтов (общий для desktop/web UI).
   SetLength(Result.RatePresets, 6);
   Result.RatePresets[0] := 48000;