feat: improved admin UI styles and layout

- Max-width 860px on editor card
- Two-column row for ServiceName/Key/Language fields
- Gradient card header, proper shadow, grey footer
- Footer buttons right-aligned
- Smaller uppercase table headers, better typography
- Overhauled admin.css with CSS variables and cleaner rules

Ref: IT-628

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

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

@@ -1,17 +1,16 @@
 @{
     Layout = "~/Views/Shared/_Layout.cshtml";
 }
-<div class="card shadow-sm">
-    <div class="card-header bg-primary text-white">
-        <h5 class="mb-0 editor-card-header">
-            <i class="bi bi-pencil-square"></i>
-            @ViewData["EditorTitle"]
-        </h5>
-    </div>
-    <div class="card-body">
-        @RenderBody()
-    </div>
-    <div class="card-footer d-flex gap-2">
-        @RenderSection("FormActions", required: false)
+<div class="editor-wrapper">
+    <div class="card editor-card">
+        <div class="card-header editor-card-header-bar">
+            <i class="bi bi-pencil-square me-2"></i>@ViewData["EditorTitle"]
+        </div>
+        <div class="card-body editor-card-body">
+            @RenderBody()
+        </div>
+        <div class="card-footer editor-card-footer">
+            @RenderSection("FormActions", required: false)
+        </div>
     </div>
 </div>