feat(templates/ui): phase 6-8 — derived template UX

Templates tree hides IsDerived templates by default. A "Show derived"
form-switch in the page header toggles them into the listing so users
can reach orphaned derived templates when they need to.

TemplateEdit:
- Banner on derived templates: links to the base + the composing owner /
  slot name pulled from OwnerCompositionId.
- Attributes/Scripts tables grew a context-aware column:
  * On derived templates: a Source badge (Inherited / Override / Local)
    plus a 🔒 Base-locked badge when the base marks LockedInDerived.
  * On base templates: a switch that flips LockedInDerived through
    UpdateAttribute/UpdateScript.
- Effective Value / Code now resolves from the base when an inherited row
  carries a stale snapshot — matches the flatten-time behavior so the UI
  doesn't lie.
- Override / Revert-to-base actions added to the row kebab; delete is
  hidden on inherited rows (the base owns those).

src/ScadaLink.CentralUI/Components/Pages/Design/Templates.razor

@@ -62,6 +62,14 @@
                         </li>
                     </ul>
                 </div>
+                <div class="form-check form-switch d-flex align-items-center gap-2 mb-0"
+                     title="Derived templates back individual composition slots and are normally hidden.">
+                    <input class="form-check-input" type="checkbox" role="switch"
+                           id="show-derived-toggle"
+                           checked="@_showDerived"
+                           @onchange="OnToggleShowDerived" />
+                    <label class="form-check-label small text-muted" for="show-derived-toggle">Show derived</label>
+                </div>
                 <button class="btn btn-outline-secondary btn-sm"
                         title="New folder at root"
                         @onclick="() => OpenNewFolderDialog(null)">+ Folder</button>
@@ -102,6 +110,7 @@
 
     private List<Template> _templates = new();
     private List<TemplateFolder> _folders = new();
+    private bool _showDerived;
 
     private bool _loading = true;
     private string? _errorMessage;
@@ -113,6 +122,12 @@
         await LoadTemplatesAsync();
     }
 
+    private void OnToggleShowDerived(ChangeEventArgs e)
+    {
+        _showDerived = e.Value is bool b && b;
+        BuildTemplateTree();
+    }
+
     private async Task LoadTemplatesAsync()
     {
         _loading = true;
@@ -172,7 +187,10 @@
         }
 
         // 3. Template nodes with composition leaves
-        foreach (var t in _templates.OrderBy(t => t.Name, StringComparer.OrdinalIgnoreCase))
+        var visibleTemplates = _showDerived
+            ? _templates
+            : _templates.Where(t => !t.IsDerived);
+        foreach (var t in visibleTemplates.OrderBy(t => t.Name, StringComparer.OrdinalIgnoreCase))
         {
             var compChildren = t.Compositions
                 .OrderBy(c => c.InstanceName, StringComparer.OrdinalIgnoreCase)