Centre a node arrived on from another screen

The viewport effect marked the selection as centred before the layout
had placed anything, so an arrival at ?node= fitted the whole flow.
It is recorded once the fit has run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013GAeg4Jo6xjZzjwy9QDWMQ

frontend/src/components/Flow/FlowEditor.tsx

@@ -677,9 +677,11 @@ function FlowEditorInner({
   const centred = useRef<string | null>(null)
   // biome-ignore lint/correctness/useExhaustiveDependencies: refit when the shape or the panel changes, not on every render.
   useEffect(() => {
+    // Recorded as centred only once the fit below has run: arriving from
+    // another screen selects before the layout has answered, and spending the
+    // selection on the early returns is what fitted the whole flow instead.
     const focus =
       selectedId && selectedId !== centred.current ? selectedId : null
-    centred.current = selectedId
     // A phone's sheet covers the canvas outright, and so does the expanded
     // editor: there is no viewport to aim.
     if (editorExpanded) return
@@ -695,6 +697,7 @@ function FlowEditorInner({
           : { ...view, duration },
       )
       fitted.current = true
+      centred.current = selectedId
     })
     return () => cancelAnimationFrame(frame)
   }, [