fix #381: Options-panel footer needs an opaque backing field

Root cause: a live-DAT probe found retail authors NO backing element
behind the Character/Chat/Config tabs' Apply/Reset/Defaults footer —
each page root has exactly five children (the row ListBox, its
scrollbar, and the three buttons) with zero direct-state media on the
root itself. Scrolled row content therefore bled through visibly
between/behind the three buttons; the bleed-through is a rendering gap
in our own composition, not a missing import.

Fix: new minimal widget UiSolidSpriteFill tiles
RetailChromeSprites.CenterFill (the SAME panel-background sprite the
Options window's own chrome already draws behind everything, not an
invented color) across the footer strip's rect, derived from the three
buttons' own resolved Top/Height and z-ordered strictly behind every
other child so it can never intercept input or occlude the buttons.
Register row AP-205 records the synthesis. Regressed by
OptionsPanelControllerTests.
Bind_SynthesizesOneOpaqueFooterBacking_PerPageWithApplyResetDefaults,
which pins exactly one backing field per page, sized from the live
button rects, z-ordered behind every sibling.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/UI/RetailUiRuntime.cs

@@ -2062,7 +2062,7 @@ public sealed class RetailUiRuntime : IDisposable
             OpenConfigureKeyboard: () => ToggleWindow(WindowNames.KeyboardConfig));
 
         Layout.OptionsPanelController? controller =
-            Layout.OptionsPanelController.Bind(layout, callbacks);
+            Layout.OptionsPanelController.Bind(layout, callbacks, _bindings.Assets.ResolveSprite);
         if (controller is null)
         {
             Console.WriteLine("[UI] options panel: required root did not build as UiTabPanel.");