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>

public/build/index.html

@@ -5,7 +5,7 @@
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     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
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-    <script type="module" crossorigin src="/build/assets/index-6y_HJqaF.js"></script>
+    <script type="module" crossorigin src="/build/assets/index-C0Yc8pEe.js"></script>
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