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/MitreMatrixModal.test.tsx

@@ -189,6 +189,41 @@ describe('MitreMatrixModal', () => {
     });
   });
 
+  it('Apply button is disabled when no techniques selected and no initial selection', async () => {
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={[]} onApply={vi.fn()} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    // Label is "Clear all" when totalSelected === 0, but it's disabled when initialSelection is also empty
+    const applyBtn = screen.getByRole('button', { name: /Clear all/i });
+    expect(applyBtn).toBeDisabled();
+  });
+
+  it('Apply button shows "Clear all" and stays enabled when initial selection is deselected', async () => {
+    const onApply = vi.fn();
+    const user = userEvent.setup();
+
+    renderWithProviders(
+      <MitreMatrixModal isOpen initialSelection={SELECTION} onApply={onApply} onCancel={vi.fn()} />,
+    );
+
+    await waitFor(() => screen.getByText('T1078'));
+
+    // Deselect T1078 (it was pre-selected)
+    const t1078Btn = screen.getAllByRole('button').find(
+      (btn) => btn.textContent?.includes('T1078') && !btn.getAttribute('aria-label'),
+    );
+    await user.click(t1078Btn!);
+
+    // Button should show "Clear all" and be enabled (user explicitly clearing the list)
+    const applyBtn = screen.getByRole('button', { name: /Clear all/i });
+    expect(applyBtn).not.toBeDisabled();
+    await user.click(applyBtn);
+    expect(onApply).toHaveBeenCalledWith([]);
+  });
+
   it('backdrop click calls onCancel', async () => {
     const onCancel = vi.fn();
     const user = userEvent.setup();