feat(dashboard): complete React cutover

tests/dashboard-assets.test.ts

@@ -11,17 +11,14 @@ describe('resolveDashboardAssets', () => {
     expect(resolveDashboardAssets(workspace, fileExists)).toEqual({
       directory: path.dirname(reactIndex),
       indexFile: reactIndex,
-      isLegacy: false,
     });
   });
 
-  it('falls back to the legacy dashboard during incremental migration', () => {
+  it('fails clearly when the production React build is missing', () => {
     const workspace = path.resolve('workspace');
 
-    expect(resolveDashboardAssets(workspace, () => false)).toEqual({
-      directory: path.join(workspace, 'public'),
-      indexFile: path.join(workspace, 'public', 'index.html'),
-      isLegacy: true,
-    });
+    expect(() => resolveDashboardAssets(workspace, () => false)).toThrow(
+      'React dashboard build is missing',
+    );
   });
 });