fix(home): "next sync" must reflect the schedule the device is *on*

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>

frontend/src/test/components/DevicePicker.test.ts

@@ -32,6 +32,7 @@ const makeDevice = (overrides: Partial<Device> = {}): Device => ({
   uniquenessWindow: 30,
   linkedAt: '2026-01-01T00:00:00Z',
   lastSeenAt: null,
+  nextPollExpectedAt: null,
   lockedImageId: null,
   currentImageId: null,
   ...overrides,