feat(ui/templates): adopt TreeView design guide; split editor to /design/templates/{id}

Templates page is now a tree-only browser; editing happens on a dedicated
TemplateEdit page. Drag-drop is replaced by context-menu Move-to-Folder.
TreeView gains Bootstrap Icons (chevron + per-kind glyphs), ancestor guide
lines, defined hover/selected/focus tokens, and Escape-dismisses-menu per
the new Visual Design Guide (V1-V7) in Component-TreeView.md.

src/ScadaLink.Host/Components/App.razor

@@ -6,6 +6,8 @@
     <base href="/" />
     <title>ScadaLink</title>
     <link href="/lib/bootstrap/css/bootstrap.min.css" rel="stylesheet" />
+    <link href="/lib/bootstrap-icons/bootstrap-icons.css" rel="stylesheet" />
+    <link href="/ScadaLink.Host.styles.css" rel="stylesheet" />
     <style>
         .sidebar {
             min-width: 220px;