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>

RadioController.pas

@@ -544,6 +544,9 @@ type
     procedure EnsureBackend(Kind: TBackendKind);
     // Возможности текущего бэкенда (для гейтинга UI / пресетов sample rate).
     function  BackendCaps: TBackendCaps;
+    // Единый источник пресетов sample-rate для UI (desktop overlay + web).
+    // Берётся из caps текущего бэкенда (HPSDR 48k..1536k / Pluto 576k..5760k).
+    function  SampleRatePresets: TBackendRateArray;
     // True если активный бэкенд — Pluto (VHF/UHF band-план вместо HF).
     function  IsPluto: Boolean;
     // Человекочитаемое имя подключённой платы для статус-строки (UI/Web).
@@ -3154,6 +3157,11 @@ begin
   else FillChar(Result, SizeOf(Result), 0);
 end;
 
+function TRadioController.SampleRatePresets: TBackendRateArray;
+begin
+  Result := BackendCaps.RatePresets;
+end;
+
 function TRadioController.IsPluto: Boolean;
 begin
   Result := (FNetwork <> nil) and (FNetwork.Caps.Kind = bkPluto);