fix(ui): #145 — importer honors ZLevel so the backdrop sits behind panels

The factory mapped ZOrder from ReadOrder only, so the gmInventoryUI
full-window backdrop (0x100001D0, ReadOrder 4) painted over the nested
panels (ReadOrder 1-3). Wire ElementDesc.ZLevel through ElementInfo /
ToInfo / Merge and fold it into ZOrder = ReadOrder - ZLevel*10000 (higher
ZLevel = further back, ReadOrder the within-layer tiebreaker). Vitals
(all ZLevel 0) are unchanged; chat (ZLevel 900) + toolbar (1,2) shift to
their dat layering — verify visually.

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

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

@@ -270,6 +270,7 @@ public static class LayoutImporter
             Right            = d.RightEdge,
             Bottom           = d.BottomEdge,
             ReadOrder        = d.ReadOrder,
+            ZLevel           = d.ZLevel,
             DefaultStateName = (defState is "Undef" or "Undefined" or "0") ? "" : defState,
         };