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/vite.config.ts

@@ -1,5 +1,4 @@
-/// <reference types="vitest" />
-import { defineConfig } from 'vite'
+import { defineConfig } from 'vitest/config'
 import vue from '@vitejs/plugin-vue'
 import { fileURLToPath, URL } from 'node:url'