refactor: address loading review findings (#18)

frontend/src/app/components/tenant-form/tenant-form.html

@@ -25,7 +25,10 @@
     @if (saveError()) {
       <div class="alert-error" role="alert">{{ saveError() }}</div>
     }
-    <fieldset [disabled]="saving()" [attr.inert]="saving() ? '' : null">
+    <p class="visually-hidden" role="status" aria-live="polite">
+      {{ saving() ? 'Enregistrement en cours.' : '' }}
+    </p>
+    <fieldset class="busy-fieldset" [disabled]="saving()" [attr.inert]="saving() ? '' : null">
       <div class="form-grid">
         <mat-form-field appearance="outline"
           ><mat-label>Prénom</mat-label
@@ -79,7 +82,7 @@
       <div class="form-actions">
         <button mat-button type="button" (click)="cancelled.emit()">Annuler</button
         ><button mat-flat-button type="submit" class="primary-action">
-          <span aria-live="polite">
+          <span>
             @if (saving()) {
               <mat-spinner diameter="18" aria-hidden="true" /> Enregistrement…
             } @else {