perf: implement background thumbnail generation and inter-post preloading

Key changes:
- Added Web Worker for background image thumbnailing with a 1MiB threshold to optimize CPU/memory usage.
- Implemented a serial task queue for memory-safe high-res image processing, preventing OOM crashes.
- Added inter-post preloading in the modal for seamless 'Previous/Next' navigation.
- Refined scanning UI with double-buffering and a dark background to completely eliminate white flashes.
- Renamed project to 'instaarchive-viewer' in package.json.
- Fixed 'Open image in new tab' by denylisting /archives and /api in PWA config.

vite.config.ts

@@ -34,6 +34,7 @@ export default defineConfig(({mode}) => {
           ]
         },
         workbox: {
+          navigateFallbackDenylist: [/^\/api/, /^\/archives/],
           globPatterns: ['**/*.{js,css,html,ico,png,svg,woff,woff2}'],
           runtimeCaching: [
             {