Local→server data migration via localStorage marker key

Cleanest pattern for adding server-side persistence to a previously-local-only feature: server is the new source of truth, but in the consuming component a one-time migration runs that reads localStorage, uploads each entry to the new API, then sets a marker key (e.g. feature-migrated: "1") so subsequent loads skip the upload. Three gotchas: (1) gate the migration on serverList.length === 0 — if the user already has server data from another device, do NOT overwrite it with local data; (2) gate on the marker key in localStorage itself, NOT in component state — a remount would otherwise re-trigger; (3) use a useRef boolean in addition to the marker key to handle the StrictMode double-invoke during the same mount before the marker write completes. For the rendering path, the simplest architecture is RTK-Query (or equivalent) as the dictionary source of truth, plus a small useEffect that mirrors the fetched data into a redux/zustand cache that other code (e.g. an applyPreset reducer) can do synchronous lookups against — keeps existing imperative code working without rewriting every call site to be async.