Add Deploy/Redeploy button and fix actor replacement on redeployment

Instances page gains Deploy button that triggers flattening pipeline and sends
config to site. Button shows "Redeploy" when instance is stale. Fixed actor name
collision on redeployment by scheduling deferred recreation after Context.Stop.

src/ScadaLink.SiteRuntime/Actors/DeploymentManagerActor.cs

@@ -180,9 +180,14 @@ public class DeploymentManagerActor : ReceiveActor, IWithTimers
         {
             Context.Stop(existing);
             _instanceActors.Remove(instanceName);
+            // Wait for the child to be removed from the children collection
+            // by yielding and retrying — Context.Stop is processed before the next message
+            Context.System.Scheduler.ScheduleTellOnce(
+                TimeSpan.FromMilliseconds(500), Self, command, Sender);
+            return;
         }
 
-        // Create the Instance Actor immediately
+        // Create the Instance Actor immediately (no existing actor to replace)
         CreateInstanceActor(instanceName, command.FlattenedConfigurationJson);
 
         // Persist to SQLite and clear static overrides asynchronously