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/assets/DevicePicker-DcPlHAXI.js

@@ -0,0 +1 @@
+import{A as e,O as t,R as n,_ as r,d as i,ft as a,g as o,h as s,l as c,o as l,p as u,t as d,u as f}from"./_plugin-vue_export-helper-CeYnMxKK.js";import{n as p,t as m}from"./BaseBottomSheet-Y2PW4H1i.js";var h={class:`device-picker__list`},g=[`checked`,`onChange`],_={class:`device-picker__name`},v={class:`device-picker__orientation`},y=d(r({__name:`DevicePicker`,props:{modelValue:{type:Boolean},devices:{},selected:{},uploading:{type:Boolean}},emits:[`update:modelValue`,`update:selected`,`confirm`],setup(r,{emit:d}){let y=r,b=d;function x(e){y.selected.includes(e)?b(`update:selected`,y.selected.filter(t=>t!==e)):b(`update:selected`,[...y.selected,e])}let S=c(()=>{let e=y.selected.length;return e===0?`Add to frame`:`Add to ${e} frame${e>1?`s`:``}`});return(c,d)=>(t(),i(m,{"model-value":r.modelValue,label:`Choose frames`,"onUpdate:modelValue":d[1]||=e=>c.$emit(`update:modelValue`,e)},{default:n(()=>[d[2]||=f(`h2`,{class:`device-picker__title`},`Add to frames`,-1),d[3]||=f(`p`,{class:`device-picker__sub`},`Choose which frames will show this photo.`,-1),f(`div`,h,[(t(!0),u(l,null,e(r.devices,e=>(t(),u(`label`,{key:e.id,class:`device-picker__row`},[f(`input`,{type:`checkbox`,class:`device-picker__check`,checked:r.selected.includes(e.id),onChange:t=>x(e.id)},null,40,g),f(`span`,_,a(e.name),1),f(`span`,v,a(e.orientation),1)]))),128))]),o(p,{variant:`primary`,class:`device-picker__confirm`,disabled:r.selected.length===0||r.uploading,onClick:d[0]||=e=>c.$emit(`confirm`)},{default:n(()=>[s(a(r.uploading?`Uploading…`:S.value),1)]),_:1},8,[`disabled`])]),_:1},8,[`model-value`]))}}),[[`__scopeId`,`data-v-a6466fa5`]]);export{y as t};