feat(D.2b): UiButton (Type 1) — Send + Max/Min as generic buttons (widget-generalization Task 3)

Introduces UiButton: a dedicated dat-widget button that ports UIElement_Button
(RegisterElementClass(1,...) @ acclient_2013_pseudo_c.txt:125828). State selection,
tiled DrawSprite, and label rendering mirror UiDatElement exactly so the chat Send
and Max/Min buttons have zero behavioral change.

DatWidgetFactory now maps Type 1 → UiButton (beside Type 7 → UiMeter, Type 11 →
UiScrollbar). ChatWindowController's Send and Max/Min bind blocks updated from
UiDatElement casts to UiButton casts; ClickThrough=false lines dropped (UiButton
is interactive by construction).

The old UiPanel.cs UiButton (a plain dev-scaffold rect+text button with no dat
sprites) is renamed UiSimpleButton to free the name — no production code
instantiated it.

Full suite: 402 passed, 2 skipped, 0 failed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/Layout/DatWidgetFactory.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Linq;
+using AcDream.App.UI;
 
 namespace AcDream.App.UI.Layout;
 
@@ -57,6 +58,7 @@ public static class DatWidgetFactory
 
         UiElement e = info.Type switch
         {
+            1  => new UiButton(info, resolve),         // UIElement_Button (reg :125828)
             7  => BuildMeter(info, resolve, datFont),  // UIElement_Meter
             11 => new UiScrollbar(),                   // UIElement_Scrollbar (reg :124137)
             _  => new UiDatElement(info, resolve),     // generic fallback for all other types