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>

WebServer.pas

@@ -300,6 +300,8 @@ type
     FRatePresets: array of Integer; // sample-rate пресеты текущего бэкенда (для span-кнопок)
     FBandNames:   array of string;  // band-план текущего бэкенда (имена кнопок)
     FBandFreqs:   array of Double;  // band-план: дефолтные частоты (для подсветки selector)
+    FFreqMin:     Double;           // диапазон перестройки бэкенда (клампинг drag/wheel)
+    FFreqMax:     Double;
     FCurrentXvtr: Integer;        // -1 = HF, иначе индекс активного XVTR
     FOnXvtrBand:  TWebCmdXvtrBand;
 
@@ -397,6 +399,7 @@ type
     procedure SetDeviceList(const ADevices: TWebDeviceArray);
     procedure SetRatePresets(const ARates: array of Integer);
     procedure SetBands(const ANames: array of string; const AFreqs: array of Double);
+    procedure SetFreqRange(AMin, AMax: Double);
     property OnSpan:   TWebCmdSpan   read FOnSpan   write FOnSpan;
     property OnVolume: TWebCmdVolume read FOnVolume write FOnVolume;
     property OnWfAGC:  TWebCmdWfAGC  read FOnWfAGC  write FOnWfAGC;
@@ -546,6 +549,7 @@ begin
   SetBands(['160m','80m','60m','40m','30m','20m','17m','15m','12m','10m','6m'],
            [1840000,3700000,5357000,7100000,10120000,14200000,
             18130000,21300000,24940000,28400000,50150000]);
+  SetFreqRange(0, 61440000);   // HPSDR bootstrap; хост перезапишет под бэкенд
 end;
 
 procedure TWebServer.SetXvtrBands(const ABands: TWebXvtrArray; ACurrent: Integer);
@@ -606,6 +610,19 @@ begin
   end;
 end;
 
+procedure TWebServer.SetFreqRange(AMin, AMax: Double);
+// Диапазон перестройки текущего бэкенда (caps): web клампит drag/wheel/click по
+// спектру в эти границы. HPSDR 0..61.44МГц / Pluto 46.875МГц..6ГГц.
+begin
+  FStateLock.Enter;
+  try
+    FFreqMin := AMin;
+    FFreqMax := AMax;
+  finally
+    FStateLock.Leave;
+  end;
+end;
+
 destructor TWebServer.Destroy;
 begin
   Stop;
@@ -1646,7 +1663,8 @@ begin
 	      '"status_text":"%s","board_text":"%s","ip_text":"%s","supply_text":"%s",' +
 	      '"pll_text":"%s","rx_text":"%s","tx_text":"%s","seq_text":"%s",' +
 	      '"xvtr_current":%d,"xvtr_bands":%s,"freq_mhz_digits":%d,' +
-	      '"fmstep_idx":%d,"devices":%s,"rate_presets":%s,"bands":%s}',
+	      '"fmstep_idx":%d,"devices":%s,"rate_presets":%s,"bands":%s,' +
+	      '"freq_min":%.0f,"freq_max":%.0f}',
       [FFreq, FVfoB, FActiveVfo,
        FMode, MODE_N[FMode mod 8],
        FFilterIdx, FFilterBW,
@@ -1675,7 +1693,8 @@ begin
 	       JsonEscape(FSupplyText), JsonEscape(FPLLText), JsonEscape(FRXText),
 	       JsonEscape(FTXText), JsonEscape(FSeqText),
 	       FCurrentXvtr, XvtrJson, FFreqMhzDigits,
-       FFMStepIdx, DevJson, RateJson, BandJson
+       FFMStepIdx, DevJson, RateJson, BandJson,
+       FFreqMin, FFreqMax
       ], FS);
   finally
     FStateLock.Leave;