fix(sandboxes): stop create mode surviving a return to an open sandbox

Cursor Bugbot. Create mode and having a sandbox open are mutually exclusive, but
nothing enforced it, so both could be set at once — and the screen then lied about
which sandbox its Delete pointed at.

With `isCreating` true and `selectedId` restored, `baseline` is null, so the editor
renders an empty "New sandbox" form, while the Delete action is built from
`selected` and still targets the restored sandbox. An admin looking at a blank
create form could delete a sandbox it never named.

Two ways in, both closed:

- Browser Forward after starting a new sandbox restores `selectedId` without going
  through `closeEditor`. The render-time sync that already drops a stale draft now
  also leaves create mode, which is the same class of correction and the reason
  that block exists.
- "New sandbox" set `isCreating` without clearing `selectedId`, so the same
  contradiction was reachable without touching history at all. It now clears the
  selection, with `history: 'replace'` because switching mode is not a destination.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: icecrasher321

apps/sim/app/workspace/[workspaceId]/settings/components/sandboxes/sandboxes.tsx

@@ -81,6 +81,11 @@ export function Sandboxes() {
       setDraft(null)
       setIssues([])
     }
+    // Creating and having one open are mutually exclusive, and history can land on
+    // a sandbox while create mode is still set — Forward after starting a new one.
+    // Leaving both on renders an empty "New sandbox" form whose Delete still points
+    // at the restored sandbox.
+    if (selectedId) setIsCreating(false)
   }
 
   const sandboxes = data?.sandboxes ?? []
@@ -260,6 +265,9 @@ export function Sandboxes() {
                 onSelect: () => {
                   setDraft(emptyDraft())
                   setIsCreating(true)
+                  // Starting a new one is not editing the open one. Replace rather
+                  // than push: this is a mode switch, not a destination.
+                  void setSelectedId(null, { history: 'replace' })
                 },
               },
             ]