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/src/components/MitreMatrixModal.tsx

@@ -333,9 +333,11 @@ export function MitreMatrixModal({
             type="button"
             className="btn-primary"
             onClick={handleApply}
-            disabled={isLoading || isError}
+            disabled={isLoading || isError || (totalSelected === 0 && initialSelection.length === 0)}
           >
-            Apply {totalSelected > 0 ? `${totalSelected} technique${totalSelected !== 1 ? 's' : ''}` : ''}
+            {totalSelected === 0
+              ? 'Clear all'
+              : `Apply ${totalSelected} technique${totalSelected !== 1 ? 's' : ''}`}
           </button>
         </div>
       </div>

frontend/src/components/SimulationList.tsx

@@ -90,6 +90,7 @@ export function SimulationList({ engagementId }: SimulationListProps): JSX.Eleme
                   <Link
                     to={`/engagements/${engagementId}/simulations/${sim.id}/edit`}
                     className="text-ink font-medium hover:underline"
+                    onClick={(e) => e.stopPropagation()}
                   >
                     {sim.name}
                   </Link>