feat(overlay): flat themed buttons + full theme support for slice flags

Share one button paint routine: extract TFlatButton.Paint into
PaintFlatButton(canvas, rect, colors) and call it from both TFlatButton
and VfoOverlay.DrawButton — flag buttons now render identically to the
left panel (flat fill + border, no gloss), using theme Btn* colors.

Theme the whole flag card: the structural CLR_* palette (card bg,
border, text, dim, accents, freq, meter bg) is now derived from the
active TAppTheme via ApplyThemeColors, so flags follow light/dark like
the rest of the UI. Status accents (TX/SPLIT) stay fixed on purpose and
use a fixed dark text over their bright badges. Theme is pushed to
flags from ApplyDarkTheme and on creation via WireOverlayEvents.

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

MainForm.pas

@@ -2853,6 +2853,11 @@ begin
   FSpecView.SetTheme(T);
   if Assigned(FWidebandView) then FWidebandView.SetTheme(T);
 
+  // Флаги слайсов (оверлеи) — цвета кнопок из темы, как левая панель.
+  if Assigned(FVfoOverlay) then FVfoOverlay.SetTheme(T);
+  for i := 0 to High(FSliceFlags) do
+    if Assigned(FSliceFlags[i]) then FSliceFlags[i].SetTheme(T);
+
   // VFO display — вызывает UpdateVfoDisplay, который использует тему
   UpdateVfoDisplay;
 end;
@@ -5451,6 +5456,8 @@ begin
   O.OnSliceSelect := OnVfoOverlaySliceSelect;
   O.OnClose       := OnVfoOverlayClose;
   O.OnInvalidate  := OnVfoOverlayInvalidate;
+  // Актуальная тема (цвета кнопок флага = как на левой панели).
+  if FLightTheme then O.SetTheme(LightTheme) else O.SetTheme(DarkTheme);
 end;
 
 function TMainForm.FindSliceFlag(SliceId: Integer): TVfoOverlay;