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>

HPSDRNetwork.pas

@@ -633,7 +633,14 @@ begin
   Result.MinSampleRate   := 48000;
   Result.MaxSampleRate   := 1536000;
   Result.SampleRateMode  := srmDiscrete;
-  Result.RatePresets     := nil;   // контроллер использует свой HPSDR-набор
+  // Бэкенд — единственный источник своих рейтов (общий для desktop/web UI).
+  SetLength(Result.RatePresets, 6);
+  Result.RatePresets[0] := 48000;
+  Result.RatePresets[1] := 96000;
+  Result.RatePresets[2] := 192000;
+  Result.RatePresets[3] := 384000;
+  Result.RatePresets[4] := 768000;
+  Result.RatePresets[5] := 1536000;
   Result.MinFreqHz       := 0;
   Result.MaxFreqHz       := 61440000;
 end;