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>

public/build/index.html

@@ -14,7 +14,7 @@
     <meta name="mobile-web-app-capable" content="yes" />
     <meta name="apple-mobile-web-app-status-bar-style" content="default" />
     <meta name="apple-mobile-web-app-title" content="pictureFrame" />
-    <script type="module" crossorigin src="/build/assets/index-B5Obd-7x.js"></script>
+    <script type="module" crossorigin src="/build/assets/index-DabTPrXi.js"></script>
     <link rel="modulepreload" crossorigin href="/build/assets/_plugin-vue_export-helper-CeYnMxKK.js">
     <link rel="stylesheet" crossorigin href="/build/assets/index-BlLBHR1q.css">
   </head>