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/test/views/HomeView.test.ts

@@ -2,7 +2,6 @@ import { describe, it, expect, vi, beforeEach } from 'vitest'
 import { mount } from '@vue/test-utils'
 import { setActivePinia, createPinia } from 'pinia'
 import { useDevicesStore } from '@/stores/devices'
-import { useUploadStore } from '@/stores/upload'
 import HomeView from '@/views/HomeView.vue'
 import type { Device } from '@/types'
 
@@ -70,6 +69,7 @@ const makeDevice = (overrides: Partial<Device> = {}): Device => ({
   linkedAt: '2026-01-01T00:00:00Z',
   lastSeenAt: null,
   lockedImageId: null,
+  currentImageId: null,
   ...overrides,
 })