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>

PlutoBackend.pas

@@ -139,7 +139,6 @@ implementation
 
 const
   PLUTO_MIN_SR = 521000;     // нижний предел AD936x sampling_frequency
-  PLUTO_MAX_SR = 61440000;
   PLUTO_MIN_HZ = 46875000;   // AD9361 нативно (LibreSDR/AntSDR); AD9363 — 325 МГц
   PLUTO_MAX_HZ = 6000000000.0;
   PLUTO_DEF_SR = 1536000;    // дефолтный rate если контроллер задал невалидный
@@ -389,8 +388,6 @@ begin
   Result.HasRFBandwidth  := True;
   Result.HasFullDuplex   := True;
   Result.MinSampleRate   := PLUTO_MIN_SR;
-  Result.MaxSampleRate   := PLUTO_MAX_SR;
-  Result.SampleRateMode  := srmContinuous;
   // Пресеты для SampleRateOverlay (все > 520 ksps, минимум AD936x).
   SetLength(Result.RatePresets, 8);
   Result.RatePresets[0] := 576000;