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fix(D.2b): arrow swap, centered menu text, scrollbar-to-top, Send caption

- scroll arrows: native sprites are opposite (0x06004C6C up / 0x06004C69 down) per live
  visual — swap the assignment, drop the V-flip.
- menu labels centered vertically in each 17px row (was top-aligned, looked corrupt).
- scrollbar pulled up to the panel top so the top arrow meets the window border and the
  max/min button lines up with it (the 6px dat offset left a gap after the resize-bar reclaim).
- Send button: the dat sprite 0x06001915 is a blank gold frame (export-confirmed), so add a
  generic optional Label/LabelFont to UiDatElement and draw "Send" centered on it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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src/AcDream.App/UI/UiChatScrollbar.cs

@@ -89,11 +89,10 @@ public sealed class UiChatScrollbar : UiElement
         // sprite (~16×32) repeats to fill the element height instead of stretch-distorting.
         DrawTiled(ctx, resolve, TrackSprite, 0f, 0f, Width, Height);
 
-        // Up button — top ButtonH rows. The dat up/down arrow sprites both point DOWN
-        // (confirmed by sprite export), so the TOP button is drawn V-FLIPPED to point UP.
-        DrawSpriteFlipV(ctx, resolve, UpSprite, 0f, 0f, Width, ButtonH);
+        // Up button — top ButtonH rows. UpSprite (0x06004C6C) is the up-arrow art, drawn 1:1.
+        DrawSprite(ctx, resolve, UpSprite, 0f, 0f, Width, ButtonH);
 
-        // Down button — bottom ButtonH rows (down arrow as-is).
+        // Down button — bottom ButtonH rows. DownSprite (0x06004C69) is the down-arrow art.
         DrawSprite(ctx, resolve, DownSprite, 0f, Height - ButtonH, Width, ButtonH);
 
         // Thumb — only when content overflows the view. Retail 3-slice: top cap +