feat: unify sample-rate presets across desktop + web UIs

Sample-rate (span) presets were duplicated in three places and none was
authoritative: HPSDR set hardcoded in the desktop overlay (SPAN_RATES),
Pluto set in backend caps, and a separate hardcoded list in the web JS.
The web showed HPSDR rates even on Pluto, and the web adapter silently
dropped any rate outside a hardcoded HPSDR whitelist (so Pluto-only rates
like 960k/2304k never switched).

Single source of truth = backend caps -> controller:
- HPSDRNetwork.Caps now fills RatePresets [48k..1536k] (was nil).
- RadioBackend: named type TBackendRateArray.
- TRadioController.SampleRatePresets returns BackendCaps.RatePresets.

Both UIs derive from it:
- Desktop: ApplyBackendCapsToUI always feeds the overlay from
  SampleRatePresets (overlay no longer owns the HPSDR list).
- Web: WebServer.SetRatePresets + rate_presets in state JSON; hosts
  (daemon OnState/startup, GUI ApplyBackendCapsToUI/startup) push
  SampleRatePresets; JS builds span buttons dynamically from it.
- WebAdapter.SyncSpan validates against SampleRatePresets instead of a
  hardcoded whitelist, so any backend rate is accepted.

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

WebAdapter.pas

@@ -199,14 +199,16 @@ procedure TWebAdapter.OnSpan(Hz: Integer);
 begin FSyncInt := Hz; FController.Invoke(@SyncSpan); end;
 
 procedure TWebAdapter.SyncSpan;
-// span ≡ sample rate. Принимаем только валидные значения (как десктоп-оверлей);
+// span ≡ sample rate. Валидируем по пресетам текущего бэкенда (единый источник,
+// как десктоп-оверлей) — иначе Pluto-рейты (960k/2304k/…) отбрасывались бы.
 // SetSampleRate сам пересоздаёт WDSP-канал + персистит (idempotent при том же rate).
-const
-  ValidSpans: array[0..5] of Integer = (48000, 96000, 192000, 384000, 768000, 1536000);
-var i: Integer;
+var
+  Presets: TBackendRateArray;
+  i: Integer;
 begin
-  for i := 0 to High(ValidSpans) do
-    if ValidSpans[i] = FSyncInt then
+  Presets := FController.SampleRatePresets;
+  for i := 0 to High(Presets) do
+    if Presets[i] = FSyncInt then
     begin
       FController.SetSampleRate(FSyncInt);
       Exit;