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>

MainForm.pas

@@ -94,9 +94,6 @@ const
 
   BAND_NAMES: array[0..BAND_COUNT-1] of string = (
     '160m','80m','60m','40m','30m','20m','17m','15m','12m','10m','6m');
-  BAND_FREQ: array[0..BAND_COUNT-1] of Double = (
-    1900000, 3750000, 5357000, 7100000, 10125000,
-    14200000, 18120000, 21200000, 24940000, 28500000, 50150000);
 
   MODE_NAMES: array[0..MODE_COUNT-1] of string = (
     'LSB','USB','DSB','CWL','CWU','FM','AM','SAM');
@@ -5397,6 +5394,7 @@ begin
     Freqs[i] := Plan[i].FreqHz;
   end;
   FWebServer.SetBands(Names, Freqs);
+  with FController.BackendCaps do FWebServer.SetFreqRange(MinFreqHz, MaxFreqHz);
 end;
 
 procedure TMainForm.PushXvtrToWeb;