feat: polished admin UI styles + email channels admin CRUD

- Extract inline styles to wwwroot/css/admin.css
- Bootstrap Icons for nav and buttons
- Styled page headers, table, empty state, readonly fields
- Email Channels admin: list, create, edit, delete
- GetAllEmailChannelsQuery + handler
- AdminChannelsController with full CRUD
- form id + form= attribute pattern for EditorLayout footer buttons

Ref: IT-628

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

HrynCo.NotificationService.Web/Views/AdminTemplates/Index.cshtml

@@ -4,9 +4,11 @@
     ViewData["Title"] = "Email Templates";
 }
 
-<div class="d-flex justify-content-between align-items-center mb-4">
-    <h2 class="mb-0">📋 Email Templates</h2>
-    <a href="/admin/templates/create" class="btn btn-primary">+ Create New Template</a>
+<div class="page-header">
+    <h2><i class="bi bi-envelope-paper"></i> Email Templates</h2>
+    <a href="/admin/templates/create" class="btn btn-primary btn-sm">
+        <i class="bi bi-plus-lg me-1"></i> Create New Template
+    </a>
 </div>
 
 @if (!ViewData.ModelState.IsValid)
@@ -21,13 +23,21 @@
 
 @if (Model is null || Model.Count == 0)
 {
-    <div class="alert alert-info">No email templates found.</div>
+    <div class="card shadow-sm table-card">
+        <div class="empty-state">
+            <i class="bi bi-envelope-paper"></i>
+            <p class="mt-2 mb-0">No email templates found.</p>
+            <a href="/admin/templates/create" class="btn btn-primary btn-sm mt-3">
+                <i class="bi bi-plus-lg me-1"></i> Create First Template
+            </a>
+        </div>
+    </div>
 }
 else
 {
-    <div class="card shadow-sm">
+    <div class="card shadow-sm table-card">
         <div class="table-responsive">
-            <table class="table table-hover mb-0">
+            <table class="table table-hover table-sm mb-0">
                 <thead class="table-dark">
                     <tr>
                         <th>Service Name</th>
@@ -47,7 +57,9 @@ else
                             <td>@t.Subject</td>
                             <td class="text-end">
                                 <a href="/admin/templates/@t.ServiceName/@t.Key/@t.LanguageCode"
-                                   class="btn btn-sm btn-outline-primary me-1">✏️ Edit</a>
+                                   class="btn btn-sm btn-outline-primary me-1">
+                                    <i class="bi bi-pencil"></i> Edit
+                                </a>
                                 <form method="post"
                                       action="/admin/templates/@t.ServiceName/@t.Key/@t.LanguageCode/delete"
                                       class="d-inline">
@@ -55,7 +67,7 @@ else
                                     <button type="submit"
                                             class="btn btn-sm btn-outline-danger"
                                             onclick="return confirm('Delete this template?')">
-                                        🗑️ Delete
+                                        <i class="bi bi-trash"></i> Delete
                                     </button>
                                 </form>
                             </td>