fix(frontend): sprint 3 post-review — real dedup test + Apply 0 guard + Link stopPropagation

- MitreTechniquesField test: rewrite dedup test to actually exercise picker
  selection path — types query, waits for option, fires pointerDown,
  asserts no PATCH sent (dedup guard in handleSelect now truly covered)
- MitreMatrixModal: Apply button disabled only when totalSelected === 0
  AND initialSelection.length === 0 (no-op case); when totalSelected === 0
  but initialSelection was non-empty, shows "Clear all" and stays enabled
  so user can explicitly wipe the list
- MitreMatrixModal tests: update disabled test to match "Clear all" label,
  add "Clear all" enabled + onApply([]) path test
- SimulationList: stopPropagation on Name <Link> to prevent double-navigate
  with row onClick handler

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

frontend/tests/MitreTechniquesField.test.tsx

@@ -110,16 +110,29 @@ describe('MitreTechniquesField', () => {
     expect(screen.getByRole('combobox')).toBeInTheDocument();
   });
 
-  it('dedup: adding an already-present technique does not PATCH', async () => {
+  it('dedup: selecting an already-present technique does not PATCH', async () => {
     mock.onGet('/mitre/techniques').reply(200, [T1059]);
     const user = userEvent.setup();
     renderWithProviders(
       <MitreTechniquesField value={[T1059]} simulationId={7} engagementId={42} />,
     );
-    // open picker
+
+    // Open the quick-search picker
     await user.click(screen.getByRole('button', { name: /Quick search/i }));
-    // Picker shows; but we can't easily select the same item without triggering real debounce in this test.
-    // Instead just verify no PATCH happened yet — dedup is the key invariant.
+    const combobox = screen.getByRole('combobox');
+    expect(combobox).toBeInTheDocument();
+
+    // Type to trigger the search (debounce is 200ms but fake timers not needed — mock responds immediately)
+    await user.type(combobox, 'T1059');
+
+    // Wait for the option to appear in the listbox
+    const option = await screen.findByRole('option', { name: /T1059/i });
+    expect(option).toBeInTheDocument();
+
+    // Select it via pointerDown (mirrors the component's onPointerDown handler)
+    await user.pointer({ target: option, keys: '[MouseLeft>]' });
+
+    // Dedup guard should have fired — no PATCH should have been sent
     expect(mock.history.patch.length).toBe(0);
   });