🎨 Palette: Improve Button loading state accessibility

- Add `aria-busy` attribute to Button when loading.
- Refactor Button rendering to keep children in DOM (visually hidden) instead of unmounting.
- Fix layout shift regression by replicating flex properties in content wrapper.
- Move inline styles to CSS modules.
- Add tests for loading state accessibility.

Co-authored-by: ragusa-it <196988693+ragusa-it@users.noreply.github.com>

src/components/ui/__tests__/Button.test.tsx

@@ -19,4 +19,13 @@ describe('Button', () => {
     const button = screen.getByTestId('custom-button');
     expect(button).toBeTruthy();
   });
+
+  it('renders loading state correctly', () => {
+    render(<Button isLoading>Submit</Button>);
+    const button = screen.getByRole('button', { name: /submit/i }) as HTMLButtonElement;
+    expect(button.getAttribute('aria-busy')).toBe('true');
+    expect(button.disabled).toBe(true);
+    // Verify text is present (opacity: 0 doesn't remove from DOM)
+    expect(screen.getByText('Submit')).toBeTruthy();
+  });
 });