feat(editor): reset-to-flat button for terrain, consolidation

- "Reset to Flat" button in Noise Generator section: zeroes all heights
  across entire tile for starting over without creating a new terrain
- Useful workflow: reset → noise → smooth → scale → clamp → auto-paint

tools/editor/editor_ui.cpp

@@ -479,8 +479,12 @@ void EditorUI::renderBrushPanel(EditorApp& app) {
                 app.getTerrainEditor().scaleHeights(hScale);
                 app.showToast("Heights scaled");
             }
+            if (ImGui::Button("Reset to Flat", ImVec2(-1, 0))) {
+                app.getTerrainEditor().resetToFlat();
+                app.showToast("Terrain reset to flat");
+            }
             ImGui::TextColored(ImVec4(0.6f, 0.6f, 0.6f, 1),
-                "Exaggerate (>1) or flatten (<1) terrain relief");
+                "Scale: exaggerate (>1) or flatten (<1) relief");
         }
 
         ImGui::Separator();