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feat(D.2b): chat polish — typing fix, opacity, scrollbar 3-slice, retail channel menu

Visual-iteration batch (decomp-grounded), each fix verified against the retail screenshots:
- typing: UiElement.HitTest aborted on ClickThrough BEFORE walking children, so the
  ClickThrough UiDatElement panels blocked hit-testing to the input/transcript inside
  them. Check ClickThrough AFTER the child walk (it only gates whether THIS element
  claims the hit). Restores input focus + typing.
- opacity: UiElement.Opacity + a UiRenderContext alpha stack applied to sprite/rect
  draws (text bypasses it, stays sharp); chat frame Opacity=0.75 → translucent chat.
- brown sliver: grow the transcript panel up 9px to cover the dropped resize-bar strip.
- scrollbar: real 3-slice thumb (caps 0x06004C60/66 + tiled mid) + tiled track.
- max/min: shifted one button-width left of the scrollbar (dat right-anchors collide).
- system text now green (retail ChatMessageType 5; was yellow).
- word-wrap: transcript lines wrap to the panel width (greedy, ports GlyphList::Recalculate).
- channel menu reworked to retail gmMainChatUI::InitTalkFocusMenu: "Chat" button + a
  TWO-COLUMN popup of the 14 talk-focus items (Squelch, Tell to Selected, Chat to All,
  Tell to Fellows, ...) on a tan panel; channel items set the active outbound channel.

Build + 392 App tests green. Visual confirmation in progress.

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

@@ -93,6 +93,11 @@ public abstract class UiElement
     /// <summary>Painter's-algorithm z-order within siblings. Higher = on top.</summary>
     public int  ZOrder          { get; set; }
 
+    /// <summary>Window opacity (0..1) multiplied into this element's and its
+    /// descendants' background + sprite draws (text stays opaque). 1 = fully opaque.
+    /// Set on a top-level window (e.g. the chat frame) for retail's translucent chat.</summary>
+    public float Opacity        { get; set; } = 1f;
+
     /// <summary>If true, a left-drag on this element (or a non-draggable child of
     /// it) repositions it as a movable window. Intended for top-level panels,
     /// whose Left/Top are screen coordinates (Root sits at the origin).</summary>
@@ -179,8 +184,10 @@ public abstract class UiElement
     {
         if (!Visible) return;
 
-        // Translate into our local space.
+        // Translate into our local space + push this window's opacity (multiplies into
+        // descendants' sprite/rect draws; text bypasses the alpha so it stays sharp).
         ctx.PushTransform(Left, Top);
+        ctx.PushAlpha(Opacity);
         try
         {
             OnDraw(ctx);
@@ -201,6 +208,7 @@ public abstract class UiElement
         }
         finally
         {
+            ctx.PopAlpha();
             ctx.PopTransform();
         }
     }
@@ -220,9 +228,14 @@ public abstract class UiElement
     /// </summary>
     internal UiElement? HitTest(float localX, float localY)
     {
-        if (!Visible || !Enabled || ClickThrough) return null;
+        if (!Visible || !Enabled) return null;
 
-        // Children first, in reverse Z-order (topmost first).
+        // Children first, in reverse Z-order (topmost first). ClickThrough means
+        // THIS element is transparent to the pointer — but its children are NOT.
+        // A ClickThrough container (e.g. a UiDatElement panel that hosts the chat
+        // input / transcript) must still let the pointer reach its behavioral
+        // children, so the ClickThrough check happens AFTER the child walk, gating
+        // only whether THIS element claims the hit.
         if (_children.Count > 0)
         {
             var ordered = _children.ToArray();
@@ -235,6 +248,7 @@ public abstract class UiElement
             }
         }
 
+        if (ClickThrough) return null;
         return OnHitTest(localX, localY) ? this : null;
     }