form/admin: don't allow "+" in username/email

Jellyfin doesn't like this.

ts/form.ts

@@ -176,13 +176,32 @@ const rePasswordField = document.getElementById("create-reenter-password") as HT
 
 let captcha = new Captcha(window.code, window.captcha, window.reCAPTCHA, false);
 
+const clearSubmitButton = () => {
+    submitInput.setCustomValidity("");
+    submitSpan.title = "";
+};
+
+const invalidMessage = (el: HTMLInputElement, msg: string) => {
+    el.setCustomValidity(msg);
+    submitInput.setCustomValidity(msg);
+    submitSpan.title = msg;
+};
+
 function _baseValidator(oncomplete: (valid: boolean) => void, captchaValid: boolean): void {
+    clearSubmitButton();
     if (window.emailRequired) {
         if (!emailField.value.includes("@")) {
             oncomplete(false);
             return;
         }
     }
+    usernameField.setCustomValidity("");
+    // Jellyfin doesn't like having "+" in the username field
+    if (usernameField.value.includes("+")) {
+        invalidMessage(usernameField, window.messages["errorSpecialSymbols"]);
+        oncomplete(false);
+        return;
+    }
     if (window.discordEnabled && window.discordRequired && !discordVerified) {
         oncomplete(false);
         return;

ts/modules/accounts.ts

@@ -1138,7 +1138,12 @@ export class accountsList {
                         console.log("User created, but welcome email failed");
                     }
                 } else {
-                    window.notifications.customError("addUser", window.lang.var("notifications", "errorUserCreated", `"${send['username']}"`));
+                    let msg = window.lang.var("notifications", "errorUserCreated", `"${send['username']}"`);
+                    if ("error" in req.response) {
+                        let realError = window.lang.notif(req.response["error"]);
+                        if (realError) msg = realError;
+                    }
+                    window.notifications.customError("addUser", msg);
                 }
                 if (req.response["error"] as String) {
                     console.log(req.response["error"]);