feat(editor): height offset tool for shifting entire terrain up/down

- Offset Heights: shifts all terrain heights by a constant amount
  (-100 to +100 range with Apply button)
- Useful for raising terrain above water level or sinking below
- Slider + Apply button in the Noise Generator section

tools/editor/editor_ui.cpp

@@ -546,6 +546,13 @@ void EditorUI::renderBrushPanel(EditorApp& app) {
                 app.getTerrainEditor().invertHeights();
                 app.showToast("Heights inverted");
             }
+            static float offsetAmt = 10.0f;
+            ImGui::SliderFloat("Offset##ht", &offsetAmt, -100.0f, 100.0f, "%.0f");
+            ImGui::SameLine();
+            if (ImGui::SmallButton("Apply##off")) {
+                app.getTerrainEditor().offsetHeights(offsetAmt);
+                app.showToast("Heights offset");
+            }
             ImGui::TextColored(ImVec4(0.6f, 0.6f, 0.6f, 1),
                 "Scale: exaggerate (>1) or flatten (<1) relief");
         }