feat(frontend): add centralized constants for enrollment limits

- Create enrollment.constants.ts with MAX_SUBJECTS, MAX_CREDITS, CREDITS_PER_SUBJECT
- Create error-messages.constants.ts with ERROR_CODES and RESTRICTION_TRANSLATIONS
- Remove empty folders: core/state, shared/directives, features/dashboard
- Use 'as const' for type-safe constants

This improves maintainability by having a single source of truth for business rules.

src/frontend/src/app/core/constants/enrollment.constants.ts

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+/**
+ * Enrollment business rules constants.
+ * These values must match the backend domain rules.
+ */
+export const ENROLLMENT_LIMITS = {
+  /** Maximum number of subjects a student can enroll in */
+  MAX_SUBJECTS: 3,
+  /** Maximum total credits a student can have */
+  MAX_CREDITS: 9,
+  /** Credits per subject (fixed value) */
+  CREDITS_PER_SUBJECT: 3,
+} as const;
+
+/** Type for enrollment limits to ensure type safety */
+export type EnrollmentLimits = typeof ENROLLMENT_LIMITS;

src/frontend/src/app/core/constants/error-messages.constants.ts

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+/**
+ * Error codes returned by the GraphQL API.
+ */
+export const ERROR_CODES = {
+  MAX_ENROLLMENTS: 'MAX_ENROLLMENTS',
+  SAME_PROFESSOR: 'SAME_PROFESSOR',
+  STUDENT_NOT_FOUND: 'STUDENT_NOT_FOUND',
+  SUBJECT_NOT_FOUND: 'SUBJECT_NOT_FOUND',
+  DUPLICATE_ENROLLMENT: 'DUPLICATE_ENROLLMENT',
+  VALIDATION_ERROR: 'VALIDATION_ERROR',
+  NETWORK_ERROR: 'NETWORK_ERROR',
+  SERVER_ERROR: 'SERVER_ERROR',
+  UNKNOWN: 'UNKNOWN',
+} as const;
+
+export type ErrorCode = (typeof ERROR_CODES)[keyof typeof ERROR_CODES];
+
+/**
+ * User-friendly error messages in Spanish.
+ * Maps API error codes to display messages.
+ */
+export const ERROR_MESSAGES: Record<ErrorCode, string> = {
+  [ERROR_CODES.MAX_ENROLLMENTS]: 'Has alcanzado el límite máximo de 3 materias',
+  [ERROR_CODES.SAME_PROFESSOR]: 'Ya tienes una materia con este profesor',
+  [ERROR_CODES.STUDENT_NOT_FOUND]: 'El estudiante no existe en el sistema',
+  [ERROR_CODES.SUBJECT_NOT_FOUND]: 'La materia seleccionada no existe',
+  [ERROR_CODES.DUPLICATE_ENROLLMENT]: 'Ya estás inscrito en esta materia',
+  [ERROR_CODES.VALIDATION_ERROR]: 'Los datos ingresados no son válidos',
+  [ERROR_CODES.NETWORK_ERROR]: 'No se pudo conectar con el servidor',
+  [ERROR_CODES.SERVER_ERROR]: 'Error interno del servidor',
+  [ERROR_CODES.UNKNOWN]: 'Ha ocurrido un error inesperado',
+};
+
+/**
+ * Translations for enrollment restriction reasons.
+ * Maps API English messages to Spanish display messages.
+ */
+export const RESTRICTION_TRANSLATIONS: 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',
+};

src/frontend/src/app/core/constants/index.ts

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+export * from './enrollment.constants';
+export * from './error-messages.constants';