fix(ui): wrap chat panel body in outer BeginChild so drag-trap covers it

The InvisibleButton drag-trap inside BeginChild only catches clicks
inside that specific child. Chat had widgets OUTSIDE the inner
##chattail child (the Copy-mode Checkbox + a Separator at top, the
footer Separator + InputTextSubmit at bottom) — empty space around
those widgets fell through directly to the parent window's
window-drag init.

Fix: wrap the entire chat panel body in a single outer ##chatbody
BeginChild before drawing any content. The renderer's drag-trap
fires inside this outer child too, absorbing every empty-space
click in the chat panel body. The inner ##chattail child is now
nested inside it, which doesn't change its scroll-tail semantics
but does mean it gets its own drag-trap as a bonus.

Test fixed: Render_BeginChild_ReservesNegativeFooterFromFrameHeight
was using Single(BeginChild) — there are now two BeginChild calls
(##chatbody outer + ##chattail inner). Switched to Single(... &&
Args[0] == "##chattail") so the test still pins the footer reserve
on the inner call where it lives.

dotnet build green; 1,309 / 1,309 tests green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.UI.Abstractions/Panels/Chat/ChatPanel.cs

@@ -90,6 +90,20 @@ public sealed class ChatPanel : IPanel
             return;
         }
 
+        // L.0 follow-up: wrap the entire chat panel body in a single
+        // outer BeginChild so empty-space clicks anywhere in the body
+        // (Checkbox row, between Separator and input, etc.) are
+        // absorbed by BeginChild's drag-trap (an InvisibleButton the
+        // ImGui renderer adds inside every BeginChild). Without this
+        // wrapper the chat panel was draggable from any empty body
+        // pixel — only the inner ##chattail area was protected.
+        if (!renderer.BeginChild("##chatbody", new System.Numerics.Vector2(0f, 0f)))
+        {
+            renderer.EndChild();
+            renderer.End();
+            return;
+        }
+
         // L.0 follow-up: top-of-panel "Copy mode" toggle. When on, the
         // chat tail rendering swaps to TextMultilineReadOnly so the
         // user can mark + Ctrl+C any text. Off (default) preserves the
@@ -186,6 +200,7 @@ public sealed class ChatPanel : IPanel
             if (TryHandleClientCommand(trimmed))
             {
                 _input = string.Empty;
+                renderer.EndChild(); // outer ##chatbody
                 renderer.End();
                 return;
             }
@@ -206,6 +221,7 @@ public sealed class ChatPanel : IPanel
                     _vm.ShowSystemMessage(
                         $"Unknown command: {verb}. Type /help for the list of supported commands.");
                     _input = string.Empty;
+                    renderer.EndChild(); // outer ##chatbody
                     renderer.End();
                     return;
                 }
@@ -225,6 +241,7 @@ public sealed class ChatPanel : IPanel
             _input = string.Empty;
         }
 
+        renderer.EndChild(); // outer ##chatbody
         renderer.End();
     }