fix: prevent browser from clearing username field on channel edit

autocomplete=off is ignored by Chrome/Firefox when a password field is
present - they clear the username value after page load. Use
autocomplete=new-password on both fields to signal a new-credential
context and prevent autofill interference.

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Web/Views/AdminChannels/Edit.cshtml

@@ -87,7 +87,7 @@
     <div class="row mb-4">
         <div class="col-md-6">
             <label asp-for="Username" class="form-label fw-semibold">Username</label>
-            <input asp-for="Username" class="form-control" autocomplete="off" />
+            <input asp-for="Username" class="form-control" autocomplete="new-password" />
             <span asp-validation-for="Username" class="text-danger small"></span>
         </div>
         <div class="col-md-6">