A stale event deleted on one device stayed undeletable on every other
session: the cached row showed up, the DELETE call returned 404 (server
already removed it), the optimistic catch-block restored the row, and
the user could never clear it.
- Treat HTTP 404 on DELETE as success — the event is already gone,
which is the state we wanted. Skip the optimistic restore.
- Re-fetch the visible range on document `visibilitychange` (mobile
app returns to foreground) and on window `focus` (desktop alt-tab),
throttled to once per 10s so rapid tab-flipping doesn't hammer the
API. Without a focus refresh, mobile only got fresh server state at
page-load and lived on stale data until a full reload.
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