fix: serve bootstrap locally, fix modal button null reference

- Replace CDN bootstrap with local /lib/bootstrap/ - CDN SRI hash was
  mismatching and blocking the script entirely (no Bootstrap = no modals)
- Fix addEventListener null error: script runs before @section FormActions
  renders the button, so use document-level event delegation instead

Ref: IT-628

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

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

@@ -158,8 +158,10 @@
     </div>
 
     <script>
-        document.getElementById('testModalBtn').addEventListener('click', function () {
+        document.addEventListener('click', function (e) {
+            if (e.target.closest('#testModalBtn')) {
                 bootstrap.Modal.getOrCreateInstance(document.getElementById('testModal')).show();
+            }
         });
 
         async function sendTestEmail(channelId) {

HrynCo.NotificationService.Web/Views/Shared/_Layout.cshtml

@@ -9,10 +9,7 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <title>@ViewData["Title"] – Notification Service</title>
-    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/css/bootstrap.min.css"
-          rel="stylesheet"
-          integrity="sha384-QWTKZyjpPEjISv5WaRU9OFeRpok6YctnYmDr5pNlyT2bRjXh0JMhjY6hW+ALEwIH"
-          crossorigin="anonymous" />
+    <link href="/lib/bootstrap/bootstrap.min.css" rel="stylesheet" />
     <link href="https://cdn.jsdelivr.net/npm/bootstrap-icons@1.11.3/font/bootstrap-icons.min.css"
           rel="stylesheet" />
     <link href="/css/admin.css" rel="stylesheet" />
@@ -48,9 +45,7 @@
         </div>
     </div>
 
-    <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/js/bootstrap.bundle.min.js"
-            integrity="sha384-YvpcrYf0tY3lHB60NNkmXc4s9bIOgUxi8T/jzmGBE+rYG8O9HP+CyEb1BQGE8B8Z"
-            crossorigin="anonymous"></script>
+    <script src="/lib/bootstrap/bootstrap.bundle.min.js"></script>
     @RenderSection("Scripts", required: false)
 </body>
 </html>