朱子纯 / vibe-erp

Closes the R2 gap: an admin can now manage users and roles
entirely from the SPA without touching curl or Swagger UI.

Backend (pbc-identity):
  - New RoleService with createRole, assignRole, revokeRole,
    findUserRoleCodes, listRoles. Each method validates
    existence + idempotency (duplicate assignment rejected,
    missing role rejected).
  - New RoleController at /api/v1/identity/roles (CRUD) +
    /api/v1/identity/users/{userId}/roles/{roleCode}
    (POST assign, DELETE revoke). All permission-gated:
    identity.role.read, identity.role.create,
    identity.role.assign.
  - identity.yml updated: added identity.role.create permission.

SPA (web/):
  - UsersPage — list with username link to detail, "+ New User"
  - CreateUserPage — username, display name, email form
  - UserDetailPage — shows user info + role toggle list. Each
    role has an Assign/Revoke button that takes effect on the
    user's next login (JWT carries roles from login time).
  - RolesPage — list with inline create form (code + name)
  - Sidebar gains "System" section with Users + Roles links
  - API client + types: identity.listUsers, getUser, createUser,
    listRoles, createRole, getUserRoles, assignRole, revokeRole

Infrastructure:
  - SpaController: added /users/** and /roles/** forwarding
  - SecurityConfiguration: added /users/** and /roles/** to the
    SPA permitAll block