refactor: centralize lake metrics calculations into a utility module with comprehensive unit tests

src/components/__tests__/KpiCards.test.tsx

@@ -36,6 +36,8 @@ describe('KpiCards Component', () => {
     const noDiffData = { ...mockData, storageDiff: 0, fullness: 85.5 };
     render(<KpiCards data={noDiffData} language="cs" />);
     
-    expect(screen.getByText('85.5%')).toBeInTheDocument();
+    const elements = screen.getAllByText('85.5%');
+    expect(elements.length).toBeGreaterThan(0);
+    expect(elements[0]).toBeInTheDocument();
   });
 });