Migration Angular 20 -> 21 : fin de la migration securite (0 vulnerabilite npm)

- ng update @angular/core@21 @angular/cli@21 (corrige GHSA-jrmj-c5cx-3cw6, XSS SVG)
- Migration automatique des templates vers le control flow natif (@if/@for, 138 fichiers)
- Correction des 5 `track` invalides generes par la migration (trackBy a 1 argument
  -> track $index) + suppression des fonctions trackBy mortes
- TypeScript 5.9, zone.js 0.15 ; npm audit : 0 vulnerabilite

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/app/shared/single-image-picker/single-image-picker.component.html

@@ -1,14 +1,15 @@
 <div class="sip">
   <div class="sip-frame" [style.aspectRatio]="aspectRatio">
-    <ng-container *ngIf="imageId; else uploadTpl">
+    @if (imageId) {
       <img [src]="contentUrl(imageId)" alt="" />
       <button type="button" class="sip-remove" (click)="remove()" title="Retirer l'image">
         <lucide-icon [img]="X" [size]="14"></lucide-icon>
       </button>
-    </ng-container>
-    <ng-template #uploadTpl>
+    } @else {
       <app-image-uploader [compact]="true" (uploaded)="onUploaded($event)"></app-image-uploader>
-    </ng-template>
+    }
   </div>
-  <small *ngIf="hint" class="sip-hint">{{ hint }}</small>
+  @if (hint) {
+    <small class="sip-hint">{{ hint }}</small>
+  }
 </div>