fix(home): preview tracks frame state even with locked images and 304 polls

Three problems were stacked:

1. The 200 serving path didn't set currentImage when a locked image was
   served (RotationService.advance bypassed). The frame got the locked
   photo; the DB kept the previous one; Home showed the old one.

2. The 304 path didn't flush at all. lastSeenAt (markSeen) was lost on
   every no-change poll, and any drift in currentImage couldn't self-heal.
   For a frame that's been locked for a while, polls cycle as 304 forever
   and the DB stays wrong indefinitely.

3. Pull-to-refresh fetched via fetchDevices(), which flips loading=true
   and replaces the cards with "Loading…" mid-fetch. The PTR spinner was
   working but users couldn't see the result of their refresh.

Fixes:
- Both 200 and 304 paths now set currentImage = $image and flush. The
  304 path becomes self-healing for any device whose currentImage drifted
  from reality (e.g., from before the 200-path fix).
- fetchDevices / fetchImages take an optional { silent: true } that
  skips toggling loading.value. PTR refresh callbacks pass silent so
  the cards stay visible during background refresh.
- HomeView also listens on visibilitychange and silently re-fetches when
  the PWA returns to foreground, so reopening the app shows current
  state without a manual pull.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/devices.ts

@@ -7,8 +7,13 @@ export const useDevicesStore = defineStore('devices', () => {
   const loading = ref(false)
   const error = ref<string | null>(null)
 
-  async function fetchDevices() {
-    loading.value = true
+  /**
+   * Fetch the device list. Pass `silent: true` from background refreshes
+   * (pull-to-refresh, visibility-change polling) so the loading spinner
+   * doesn't blink and replace the existing cards mid-fetch.
+   */
+  async function fetchDevices(opts: { silent?: boolean } = {}) {
+    if (!opts.silent) loading.value = true
     error.value = null
     try {
       const res = await fetch('/api/devices')