refactor: remove username from User model, login by email only

tests/unit/actions/user.messages.test.ts

@@ -6,11 +6,10 @@ import { es } from "@/i18n/dictionaries/es"
 const actionCopy = es.admin.users.actions
 
 describe("user action message localization", () => {
-  it("localizes all 6 known use-case error strings to dictionary keys", () => {
+  it("localizes all 5 known use-case error strings to dictionary keys", () => {
     expect(
       localizeUserFieldErrors(
         {
-          username: ["Username already exists"],
           email: ["Email already exists"],
           id: [
             "User not found",
@@ -22,7 +21,6 @@ describe("user action message localization", () => {
         actionCopy,
       ),
     ).toEqual({
-      username: [actionCopy.duplicateUsername],
       email: [actionCopy.duplicateEmail],
       id: [
         actionCopy.notFound,