The card's "next sync" was computed locally as `lastSeenAt + interval`,
which broke the moment the user PATCHed a new interval: the device is
still asleep on whatever schedule was active at its last poll, but the
local record now has the new interval, so we'd display a misleading
"in 2m" after a 5→3 min change.
Fix: server stamps `nextPollExpectedAt` on every poll (200/304/204),
PWA reads it directly. The timestamp doesn't move when settings are
edited — only when the device actually polls and picks up a new
schedule. Same field also drives the settings-sheet "Next update"
preview, which had the same flaw.
Side effects:
- `markSeen()` now flushes on the 204 paths too — they previously
set lastSeenAt without flushing (latent bug for devices with no
approved images / missing assets).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
