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/MitreTechniquePicker.test.tsx

@@ -219,6 +219,26 @@ describe('MitreTechniquePicker', () => {
     expect(screen.queryByRole('listbox')).toBeNull();
   });
 
+  it('typing while techniqueId is null does not reset inputValue between keystrokes', async () => {
+    mock.onGet('/mitre/techniques').reply(200, []);
+    const user = userEvent.setup({ advanceTimers: (ms) => vi.advanceTimersByTime(ms) });
+
+    renderWithProviders(
+      <MitreTechniquePicker
+        techniqueId={null}
+        techniqueName={null}
+        onChange={vi.fn()}
+      />,
+    );
+
+    const input = screen.getByRole('combobox') as HTMLInputElement;
+    await user.click(input);
+    await user.type(input, 'T10');
+
+    // Input must retain the full typed value — no mid-stroke reset
+    expect(input.value).toBe('T10');
+  });
+
   it('shows inline error when API returns 503', async () => {
     mock.onGet('/mitre/techniques').reply(503, { error: 'mitre bundle not loaded' });
     const user = userEvent.setup({ advanceTimers: (ms) => vi.advanceTimersByTime(ms) });