feat: show currently selected image on home screen frame card

Decode the device's rendered 4bpp Spectra-6 .bin into a PNG (cached
next to the .bin) so the home-screen preview matches the dithered
6-color output the e-ink actually displays.

- New endpoint: GET /api/devices/{id}/preview
- Expose currentImageId on device JSON
- HomeView passes preview URL to FrameCard for both single and compact layouts
- Drive-by: fix vite.config.ts to import defineConfig from vitest/config
  so the build no longer fails on the unknown `test` property; remove
  unused useUploadStore import in HomeView test

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

frontend/src/views/HomeView.vue

@@ -29,6 +29,7 @@
         size="large"
         :status="deviceStatus(devicesStore.devices[0])"
         :orientation="devicesStore.devices[0].orientation"
+        :thumbnailUrl="previewUrl(devicesStore.devices[0])"
         @add-photo="onAddPhoto"
         @edit="onEdit"
       />
@@ -44,6 +45,7 @@
         size="compact"
         :status="deviceStatus(device)"
         :orientation="device.orientation"
+        :thumbnailUrl="previewUrl(device)"
         @add-photo="onAddPhoto"
         @edit="onEdit"
       />
@@ -109,6 +111,11 @@ function deviceStatus(device: Device): 'ok' | 'offline' {
   const windowMs = Math.max(device.rotationIntervalMinutes * 2 * 60_000, 30 * 60_000)
   return seenMs <= windowMs ? 'ok' : 'offline'
 }
+
+function previewUrl(device: Device): string | undefined {
+  const imageId = device.lockedImageId ?? device.currentImageId
+  return imageId ? `/api/devices/${device.id}/preview?v=${imageId}` : undefined
+}
 import FrameCard from '@/components/FrameCard.vue'
 import BaseBottomSheet from '@/components/BaseBottomSheet.vue'
 import BaseButton from '@/components/BaseButton.vue'