fix(frontend): sprint 2 review fixes — MitrePicker input retention + SPA navigation

- MitreTechniquePicker: use hasHydratedFromProps ref so onChange(null,null) on
  keystrokes does not propagate back and wipe inputValue mid-stroke
- SimulationList: replace window.location.href with useNavigate(); drop
  redundant stopPropagation on inner Link
- SimulationFormPage: hoist canSaveSoc flag; replace duplicated ternary
  expressions at onSubmit and button visibility guard
- SimulationFormPage: drop dead .replace(' ', 'T') on executed_at (isoformat
  always emits 'T')
- Tests: add regression for MitrePicker input retention and SimulationList
  SPA navigation (63 tests total)

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

frontend/tests/SimulationList.test.tsx

@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
-import { screen, waitFor } from '@testing-library/react';
+import { screen, waitFor, fireEvent } from '@testing-library/react';
 import MockAdapter from 'axios-mock-adapter';
 import { apiClient } from '@/api/client';
 import { SimulationList } from '@/components/SimulationList';
@@ -104,6 +104,25 @@ describe('SimulationList — admin/redteam', () => {
     });
     expect(screen.getByTestId('new-simulation-btn')).toBeInTheDocument();
   });
+
+  it('clicking a row uses SPA navigation and does not trigger window.location change', async () => {
+    mock.onGet('/engagements/42/simulations').reply(200, SIMULATIONS);
+    const originalHref = window.location.href;
+
+    renderWithProviders(<SimulationList engagementId={42} />, {
+      routerProps: { initialEntries: ['/engagements/42'] },
+    });
+
+    await waitFor(() => {
+      expect(screen.getByText('Lateral movement test')).toBeInTheDocument();
+    });
+
+    const row = screen.getByText('Lateral movement test').closest('tr') as HTMLElement;
+    fireEvent.click(row);
+
+    // window.location.href must be unchanged (no full-page reload)
+    expect(window.location.href).toBe(originalHref);
+  });
 });
 
 describe('SimulationList — SOC role (no edit button)', () => {