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>

RadioBackend.pas

@@ -28,7 +28,6 @@ uses
 
 type
   TBackendKind    = (bkHPSDR, bkPluto);
-  TSampleRateMode = (srmDiscrete, srmContinuous);
   TBackendRateArray = array of Integer;   // пресеты sample-rate (общий тип UI)
 
   // Описание возможностей бэкенда. Контроллер гейтит UI/поведение по этим полям.
@@ -45,8 +44,6 @@ type
     HasRFBandwidth:  Boolean;   // настройка аналоговой RF-полосы (Pluto)
     HasFullDuplex:   Boolean;   // одновременный RX+TX (Pluto / QO-100)
     MinSampleRate:   Integer;
-    MaxSampleRate:   Integer;
-    SampleRateMode:  TSampleRateMode;
     RatePresets:     TBackendRateArray;   // пресеты для SampleRateOverlay
     MinFreqHz:       Double;
     MaxFreqHz:       Double;