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>

RadioBackend.pas

@@ -29,6 +29,7 @@ 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.
   TBackendCaps = record
@@ -46,7 +47,7 @@ type
     MinSampleRate:   Integer;
     MaxSampleRate:   Integer;
     SampleRateMode:  TSampleRateMode;
-    RatePresets:     array of Integer;   // пресеты для SampleRateOverlay
+    RatePresets:     TBackendRateArray;   // пресеты для SampleRateOverlay
     MinFreqHz:       Double;
     MaxFreqHz:       Double;
   end;