zustand's JSON storage serialises the whole partialized slice and
writes it to AsyncStorage on every set(), including ones that only
flip `loading`. Opening one unread message rewrote the mail cache
about five times.
`createPersistStorage` now coalesces writes per store:
- a slice whose fields are the same references as the last one is
dropped without serialising; the email and contacts slices are
built through `memoizeSlice`, so a set() that only touches UI flags
hands over the same slice
- other changes wait up to 1 s and only the newest is serialised, and
JSON identical to the stored row, or to the row just hydrated from,
isn't written again
- pending writes are flushed when the app leaves the foreground and
when an account is signed out
- the account registry is still written straight away, since the
headless push task reads it from AsyncStorage
- "Refresh cached data" removes the rows through the stores, so their
storage forgets the old JSON and writes the re-fetched data back
Linus Rath