Import and use terrain brush utilities in App.tsx and update viewport-host.ts

src/app/App.tsx

@@ -81,6 +81,16 @@ import {
   resolveSelectionActiveId,
   type EditorSelection
 } from "../core/selection";
+import {
+  clampTerrainBrushFalloff,
+  clampTerrainBrushRadius,
+  clampTerrainBrushStrength,
+  createDefaultTerrainBrushSettings,
+  getTerrainBrushToolLabel,
+  type ArmedTerrainBrushState,
+  type TerrainBrushStrokeCommit,
+  type TerrainBrushTool
+} from "../core/terrain-brush";
 import {
   createTransformSession,
   doesTransformSessionChangeTarget,