fix(frontend): i18n reactive badges + aria-labels + ExportEngagementButton + noMatch (review polish)

- ExportEngagementButton: full i18n (export.btn, export.btnLoading, export.aria)
- Layout: aria-label nav.brandHome on brand link, nav.themeAria on theme button
- SimulationList: aria-label simulation.list.rowActions on chevron button
- C2TasksPanel: aria-label c2.tasks.col.expand on expand column header
- ImportC2HistoryModal: aria-label common.select on select column header
- MitreMatrixModal: common.collapse/expand interpolated aria-label + mitre.matrix.noMatch
- fr.json: new keys common.expand/collapse/select, nav.brandHome/themeAria, export.*, simulation.list.rowActions, mitre.matrix.noMatch, c2.tasks.col.expand

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/tests/ExportEngagementButton.test.tsx

@@ -47,7 +47,7 @@ describe('ExportEngagementButton', () => {
     renderWithProviders(<ExportEngagementButton engagementId={1} />);
     expect(screen.getByTestId('export-btn')).toBeInTheDocument();
     expect(screen.getByTestId('export-dropdown-toggle')).toBeInTheDocument();
-    expect(screen.getByText('Export')).toBeInTheDocument();
+    expect(screen.getByText('Exporter')).toBeInTheDocument();
   });
 
   it('clicking primary opens dropdown with three formats', async () => {

frontend/tests/MitreMatrixModal.test.tsx

@@ -224,7 +224,7 @@ describe('MitreMatrixModal', () => {
     await waitFor(() => screen.getByText('T1078'));
     expect(screen.queryByText(/Default Accounts/)).toBeNull();
 
-    const expandBtn = screen.getByRole('button', { name: /Expand T1078/i });
+    const expandBtn = screen.getByRole('button', { name: /Déplier T1078/i });
     await user.click(expandBtn);
 
     expect(screen.getByText(/Default Accounts/)).toBeInTheDocument();