Fix topology filtering: use global context, right side drawer, edge filter

- Remove local filter bar (Search/Type/Health) — use global header
  context (Region/Env/Stage) as single source of filtering
- Pass global context selections to layout API — graph re-fetches and
  ElkSharp re-lays out only the filtered environments
- Fix backend: skip promotion edges where source or target environment
  is outside the filtered set (was causing 400 errors)
- Replace below-graph detail zone with 360px right side drawer that
  doesn't scroll away from the graph
- Stats badges (regions/environments/paths) moved to overlay on graph

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Platform/StellaOps.Platform.WebService/Services/TopologyLayoutService.cs

@@ -107,14 +107,26 @@ public sealed class TopologyLayoutService
             });
         }
 
+        var envNodeIds = new HashSet<string>(
+            environments.Select(e => $"env-{e.EnvironmentId}"),
+            StringComparer.Ordinal);
+
         foreach (var path in paths)
         {
+            var sourceNodeId = $"env-{path.SourceEnvironmentId}";
+            var targetNodeId = $"env-{path.TargetEnvironmentId}";
+
+            if (!envNodeIds.Contains(sourceNodeId) || !envNodeIds.Contains(targetNodeId))
+            {
+                continue;
+            }
+
             var gateLabel = BuildGateLabel(path, gatesByProfile);
             elkEdges.Add(new ElkEdge
             {
                 Id = $"path-{path.PathId}",
-                SourceNodeId = $"env-{path.SourceEnvironmentId}",
-                TargetNodeId = $"env-{path.TargetEnvironmentId}",
+                SourceNodeId = sourceNodeId,
+                TargetNodeId = targetNodeId,
                 Kind = "promotion",
                 Label = gateLabel,
             });