Add configurable VFO frequency range with GHz support

- Settings > Display: new "VFO Display" control to select max frequency
  range (999 MHz / 9.999 GHz / 99.999 GHz), persisted as freq_mhz_digits
- FreqDisplay: extend digit map [0..10] to support up to 11 digit positions;
  keyboard navigation adapts to MinMhzDigits dynamically
- MainForm/LayoutTopVfoBlock: VFO panel width scales automatically with
  selected digit count using dynamic text measurement
- Web VFO: fix broken HTML (class attr not closed before data-s — wheel and
  hover were completely non-functional); rewrite fmtVfo to use variable MHz
  digit count synced from server state; fix missing </span> for .vfo-hz
- WebServer: publish freq_mhz_digits in state JSON; updated from
  ApplyFreqMhzDigits so desktop and web stay in sync
- Fix Int32 overflow on frequencies above ~2.15 GHz: CATEngine.Pad V param
  Int64, FormatFreq/FormatFreqSV Mhz var Int64 (Mhz*1000000 overflowed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CATEngine.pas

@@ -88,7 +88,7 @@ type
     FCtx:  TCATContext;
     FLock: TCriticalSection;
 
-    function Pad(V, W: Integer): string;
+    function Pad(V: Int64; W: Integer): string;
     function FreqToStr(Hz: Double): string;
     function StrToFreq(const S: string): Double;
     function ModeToK(M: Integer): Integer;
@@ -311,7 +311,7 @@ begin if B then Result := '1' else Result := '0'; end;
 
 { ── Helpers ──────────────────────────────────────────────────────────────── }
 
-function TCATEngine.Pad(V, W: Integer): string;
+function TCATEngine.Pad(V: Int64; W: Integer): string;
 begin
   Result := IntToStr(Abs(V));
   while Length(Result) < W do Result := '0' + Result;