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/UiButton.cs

@@ -0,0 +1,111 @@
+using System;
+using System.Numerics;
+using AcDream.App.UI.Layout;
+
+namespace AcDream.App.UI;
+
+/// <summary>
+/// Generic dat-widget button — the production replacement for any dat element of
+/// Type 1 (UIElement_Button, registered via RegisterElementClass(1, UIElement_Button::Create)
+/// @ acclient_2013_pseudo_c.txt:125828).
+///
+/// <para>
+/// Draws per-state sprite media exactly like <see cref="UiDatElement"/> (same
+/// <c>ActiveState</c> defaulting, same <c>ActiveMedia()</c> fallback chain, same tiled
+/// <c>DrawSprite</c> call with UV-repeat so chrome edges tile correctly) plus an
+/// optional centered text label. The click behavior mirrors <see cref="UiDatElement"/>
+/// one-for-one so the chat Send and Max/Min buttons that previously bound through
+/// <c>UiDatElement.OnClick</c> continue to work without behavioral change.
+/// </para>
+///
+/// <para>
+/// State selection: picks <see cref="ElementInfo.DefaultStateName"/> if set, then
+/// "Normal" if the element has a Normal state sprite, then falls back to the unnamed
+/// DirectState ("" key) — identical to <see cref="UiDatElement"/>.
+/// </para>
+///
+/// <para>
+/// Built by <see cref="DatWidgetFactory"/> for Type-1 elements (chat Send 0x10000019,
+/// Max/Min 0x1000046F). NOT the same as <see cref="UiSimpleButton"/>, which is an
+/// earlier dev-scaffold widget with no dat sprites.
+/// </para>
+/// </summary>
+public sealed class UiButton : UiElement
+{
+    private readonly ElementInfo _info;
+    private readonly Func<uint, (uint tex, int w, int h)> _resolve;
+
+    /// <summary>Optional click handler. Wired by the controller (e.g. chat Submit, ToggleMaximize).</summary>
+    public Action? OnClick { get; set; }
+
+    /// <summary>Optional centered text label drawn over the sprite (e.g. "Send" on a blank gold frame).</summary>
+    public string? Label { get; set; }
+
+    /// <summary>Dat font for <see cref="Label"/>. Required for the label to draw.</summary>
+    public UiDatFont? LabelFont { get; set; }
+
+    /// <summary>Label color (default white).</summary>
+    public Vector4 LabelColor { get; set; } = Vector4.One;
+
+    /// <summary>
+    /// Active state name, runtime-settable (e.g. Max/Min toggling Normal ↔ Minimized).
+    /// Matches <see cref="UiDatElement.ActiveState"/>.
+    /// </summary>
+    public string ActiveState { get; set; } = "";
+
+    /// <param name="info">Merged <see cref="ElementInfo"/> for this element.</param>
+    /// <param name="resolve">Dat file-id → (GL texture handle, native px width, native px height).
+    /// Returns (0,0,0) when the texture is not yet uploaded.</param>
+    public UiButton(ElementInfo info, Func<uint, (uint tex, int w, int h)> resolve)
+    {
+        _info    = info;
+        _resolve = resolve;
+        ClickThrough = false;   // buttons are interactive — opt OUT of click-through
+
+        // State defaulting matches UiDatElement exactly:
+        // DefaultStateName wins; else "Normal" if that state has a sprite; else DirectState ("").
+        if (!string.IsNullOrEmpty(info.DefaultStateName))
+            ActiveState = info.DefaultStateName;
+        else if (info.StateMedia.ContainsKey("Normal"))
+            ActiveState = "Normal";
+        // else ActiveState stays "" (DirectState)
+    }
+
+    /// <summary>
+    /// Returns the File id for the current <see cref="ActiveState"/>, falling back to
+    /// the DirectState ("" key) if the named state is absent.
+    /// Returns 0 if neither exists.
+    /// Mirrors <see cref="UiDatElement.ActiveMedia()"/>.
+    /// </summary>
+    private uint ActiveFile()
+        => _info.StateMedia.TryGetValue(ActiveState, out var m) ? m.File
+         : _info.StateMedia.TryGetValue("", out var d) ? d.File : 0u;
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        uint file = ActiveFile();
+        if (file != 0)
+        {
+            var (tex, tw, th) = _resolve(file);
+            if (tex != 0 && tw != 0 && th != 0)
+            {
+                // Tiled draw — same call shape as UiDatElement.OnDraw (UV-repeat; GL_REPEAT-wrapped
+                // UI texture). Matches ImgTex::TileCSI; no Stretch mode exists.
+                ctx.DrawSprite(tex, 0, 0, Width, Height, 0, 0, Width / tw, Height / th, Vector4.One);
+            }
+        }
+
+        if (Label is { Length: > 0 } label && LabelFont is { } lf)
+        {
+            float tx = (Width  - lf.MeasureWidth(label)) * 0.5f;
+            float ty = (Height - lf.LineHeight)           * 0.5f;
+            ctx.DrawStringDat(lf, label, tx, ty, LabelColor);
+        }
+    }
+
+    public override bool OnEvent(in UiEvent e)
+    {
+        if (e.Type == UiEventType.Click && OnClick is not null) { OnClick(); return true; }
+        return false;
+    }
+}