feat: convert namespace detail to right-side drawer panel

Replace the standalone namespace detail route with an inline drawer panel
that slides in from the right when clicking a namespace in the list view.
This provides a more fluid UX without full page navigation.

Changes:
- Namespace detail now opens in a fixed-position right-side panel (600px width)
- Added semi-transparent backdrop that closes the panel when clicked
- Converted namespace links to buttons with proper click handlers
- Removed /polaris/ns/:namespace route and NamespaceDetailView import
- Updated tests to check for buttons instead of links
- Panel includes close button (×) in header

Technical details:
- Uses React state (selectedNamespace) instead of route params
- Panel styled with fixed positioning, z-index layering, and box shadow
- Backdrop at z-index 1100, panel at 1200 to overlay content
- No MUI imports (stays within Headlamp CommonComponents constraint)

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

src/components/NamespacesListView.test.tsx

@@ -117,7 +117,7 @@ describe('NamespacesListView', () => {
     expect(screen.getByText('No Polaris audit results found.')).toBeInTheDocument();
   });
 
-  it('renders namespace rows with correct scores and links', () => {
+  it('renders namespace rows with correct scores and buttons', () => {
     const data = makeAuditData([
       makeResult({
         Name: 'deploy-a',
@@ -157,12 +157,14 @@ describe('NamespacesListView', () => {
 
     renderWithRouter(<NamespacesListView />);
 
-    // Namespace links
-    const alphaLink = screen.getByText('alpha');
-    expect(alphaLink.closest('a')).toHaveAttribute('href', '/polaris/ns/alpha');
+    // Namespace buttons (now buttons instead of links for drawer)
+    const alphaButton = screen.getByText('alpha');
+    expect(alphaButton).toBeInTheDocument();
+    expect(alphaButton.tagName).toBe('BUTTON');
 
-    const betaLink = screen.getByText('beta');
-    expect(betaLink.closest('a')).toHaveAttribute('href', '/polaris/ns/beta');
+    const betaButton = screen.getByText('beta');
+    expect(betaButton).toBeInTheDocument();
+    expect(betaButton.tagName).toBe('BUTTON');
   });
 
   it('uses correct scoreStatus: >=80 success, >=50 warning, <50 error', () => {