fix(frontend): keep the persisted flow when the header list refreshes (#14659)

The app header polls GET /flows/?get_all=true&header_flows=true, whose rows
are FlowHeader objects: they carry no updated_at at all, and data is nulled
for anything that is not a component. setFlows took the matching row
wholesale as flowsManagerStore.currentFlow, which is the *persisted* flow
the editor diffs the canvas against.

Every flow open triggers that refetch (useGetFlow's onSettled refetches the
header query), so it lands right after applyFlowToCanvas has stored the full
flow — replacing it with a header. The exit dialog reads updated_at for its
"Last saved" line and so reported "Last saved: Never" for a flow that was
persisted, including after a full page reload.

Merge instead: apply the fields the refreshed row carries, keep data and
updated_at when it does not.