Lowercase usernames without the browser locale

`toLocaleLowerCase()` follows the browser locale, so with a Turkish locale `I` became a dotless `ı`, which is not a valid localpart and does not match what the server does with `to_lowercase`.

frontend/src/components/RegisterForm/RegisterFields.tsx

@@ -36,7 +36,7 @@ export const UsernameField: React.FC<{
         value={username}
         onChange={(e) => {
           // Lowercase as the user types, so what is shown is what gets sent
-          setUsername(e.target.value.toLocaleLowerCase());
+          setUsername(e.target.value.toLowerCase());
         }}
         onBlur={() => setUsername(normalizeUsername(username))}
       />

frontend/src/entrypoints/templates.ts

@@ -1,4 +1,5 @@
-/* Copyright 2025 New Vector Ltd.
+/* Copyright 2025, 2026 Element Creations Ltd.
+ * Copyright 2025 New Vector Ltd.
  *
  * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
  * Please see LICENSE files in the repository root for full details.
@@ -66,7 +67,7 @@ function patchUsernameInput(inputElement: HTMLInputElement) {
 
   inputElement.addEventListener("input", () => {
     // Simply lowercase things automatically, as this is not too disruptive
-    inputElement.value = inputElement.value.toLocaleLowerCase();
+    inputElement.value = inputElement.value.toLowerCase();
 
     const match = inputElement.value.match(VALID_USERNAME_RE);
     if (!inputElement.value.trim() || match !== null) {

frontend/src/utils/registration.ts

@@ -37,7 +37,7 @@ export const VALID_LOCALPART_RE = /^[a-z0-9._=/+-]+$/;
 
 /** Normalize a username: trim and lowercase. */
 export const normalizeUsername = (value: string): string =>
-  value.trim().toLocaleLowerCase();
+  value.trim().toLowerCase();
 
 /**
  * Well-known policy violation codes, mapped to the key of their translated