fix(frontend): translate enrollment restriction messages to Spanish

Add translateReason() method to convert API messages to Spanish:
- "Already enrolled" -> "Ya inscrito"
- "Already have a subject with this professor" -> "Ya tienes una materia con este profesor"
- "Maximum 3 subjects reached" -> "Máximo 3 materias alcanzado"

Apply translation to both warning text and tooltip.

Fixes DEF-002: Messages were displaying in English instead of Spanish,
breaking UI language consistency.

src/frontend/src/app/features/enrollment/pages/enrollment-page/enrollment-page.component.ts

@@ -142,7 +142,7 @@ import { LoadingSpinnerComponent, EmptyStateComponent } from '@shared/index';
                     @if (!item.isAvailable && item.unavailableReason) {
                       <div class="subject-card-warning">
                         <mat-icon>warning</mat-icon>
-                        {{ item.unavailableReason }}
+                        {{ translateReason(item.unavailableReason) }}
                       </div>
                     }
                   </div>
@@ -150,7 +150,7 @@ import { LoadingSpinnerComponent, EmptyStateComponent } from '@shared/index';
                     class="btn btn-sm btn-primary"
                     (click)="enroll(item)"
                     [disabled]="!item.isAvailable || processingId() === item.id || enrollments().length >= 3"
-                    [matTooltip]="!item.isAvailable ? item.unavailableReason || '' : 'Inscribir'"
+                    [matTooltip]="!item.isAvailable ? translateReason(item.unavailableReason || '') : 'Inscribir'"
                     data-testid="btn-enroll-subject"
                   >
                     @if (processingId() === item.id) {
@@ -428,4 +428,14 @@ export class EnrollmentPageComponent implements OnInit {
       },
     });
   }
+
+  private readonly reasonTranslations: Record<string, string> = {
+    'Already enrolled': 'Ya inscrito',
+    'Already have a subject with this professor': 'Ya tienes una materia con este profesor',
+    'Maximum 3 subjects reached': 'Máximo 3 materias alcanzado',
+  };
+
+  translateReason(reason: string): string {
+    return this.reasonTranslations[reason] ?? reason;
+  }
 }