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/components/DevicePicker.test.ts

@@ -33,6 +33,7 @@ const makeDevice = (overrides: Partial<Device> = {}): Device => ({
   linkedAt: '2026-01-01T00:00:00Z',
   lastSeenAt: null,
   lockedImageId: null,
+  currentImageId: null,
   ...overrides,
 })