feat(editor): complete importOpen, keyboard shortcuts, DBC exporter

- importOpen now loads WHM alpha maps + full WOT metadata (textures,
  layers, holes, water) — was height-only stub
- DBC exporter migrated to nlohmann/json (last naive JSON file)
- Add Z-axis gizmo constraint (Z key) alongside X/Y
- Add Ctrl+Y as alternate redo binding
- Add F1 keyboard shortcut for help panel toggle
- Update help panel: document Z-axis, Ctrl+Y, all shortcuts

tools/editor/editor_ui.hpp

@@ -20,6 +20,7 @@ public:
     void processActions(EditorApp& app);
     void openNewTerrainDialog() { showNewDialog_ = true; }
     void openLoadDialog() { showLoadDialog_ = true; }
+    void toggleHelp() { showHelp_ = !showHelp_; }
 
     PaintMode getPaintMode() const { return paintMode_; }