feat(frontend): sprint 4 — dark mode + matrix overhaul + tactic selection + done read-only + UI polish

US-17: fix duplicate "Create engagement" button, icon conventions (Save/RotateCcw/Grid2x2), UsersAdminPage form baseline alignment
US-18: done status fully read-only + Reopen button (done → review_required) for all roles
US-19: invalidate engagement queries on simulation PATCH/transition for auto-status propagation
US-20: MitreMatrixModal rewritten — CSS grid 12-column layout, no horizontal scroll, attack.mitre.org compact look
US-21: tactic header clickable in matrix, tactic chips (MitreTacticTag) in field, single atomic PATCH with technique_ids + tactic_ids
US-22: MitreTechniquesField chips-only area + inline search input + matrix icon button; chips show ID-only (name in title=)
US-23: useTheme hook — 3-state light/dark/system, CSS variables, Tailwind darkMode class, localStorage persistence

92/92 tests passing, typecheck and lint clean.

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

frontend/src/pages/EngagementsListPage.tsx

@@ -24,9 +24,9 @@ export function EngagementsListPage(): JSX.Element {
     if (!window.confirm(`Delete engagement "${eng.name}"? This cannot be undone.`)) return;
     try {
       await deleteMutation.mutateAsync(eng.id);
-      push('Engagement supprimé', 'success');
+      push('Engagement deleted', 'success');
     } catch (err) {
-      push(extractApiError(err, 'Suppression impossible'), 'error');
+      push(extractApiError(err, 'Could not delete engagement'), 'error');
     }
   };
 
@@ -41,7 +41,7 @@ export function EngagementsListPage(): JSX.Element {
         </div>
         {canEditEngagements ? (
           <Link to="/engagements/new" className="btn-primary">
-            New engagement
+            + New
           </Link>
         ) : null}
       </header>
@@ -59,7 +59,7 @@ export function EngagementsListPage(): JSX.Element {
           action={
             canEditEngagements ? (
               <Link to="/engagements/new" className="btn-primary">
-                Create engagement
+                + New engagement
               </Link>
             ) : undefined
           }