feat: cambiar clave desde la app (usuario logueado)

Nuevo PUT /api/auth/change-password: exige la clave actual (a diferencia de
reset-password, que usa un token de correo sin sesion). Opcion "Cambiar clave"
agregada al menu de usuario (popup), disponible para cualquier rol.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/services/auth.js

@@ -4,3 +4,4 @@ export const register = (data) => api.post('/auth/register', data);
 export const verifyEmail = (token) => api.get(`/auth/verify-email?token=${encodeURIComponent(token)}`);
 export const forgotPassword = (email) => api.post('/auth/forgot-password', { email });
 export const resetPassword = (token, password) => api.post('/auth/reset-password', { token, password });
+export const changePassword = (currentPassword, newPassword) => api.put('/auth/change-password', { currentPassword, newPassword });