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.

src/lib/thumbnail-worker.ts

@@ -0,0 +1,42 @@
+/**
+ * Thumbnail Generation Worker
+ * Uses OffscreenCanvas and createImageBitmap for high-performance,
+ * background-thread image resizing.
+ */
+
+self.onmessage = async (e: MessageEvent) => {
+  const { id, blob, width } = e.data;
+
+  try {
+    // 1. Create a bitmap from the blob (native browser decoding)
+    // We resize it DURING the decode step for maximum efficiency
+    const bitmap = await createImageBitmap(blob, {
+      resizeWidth: width,
+      resizeQuality: 'medium'
+    });
+
+    // 2. Use OffscreenCanvas to draw the resized bitmap
+    const canvas = new OffscreenCanvas(bitmap.width, bitmap.height);
+    const ctx = canvas.getContext('2d');
+    
+    if (!ctx) {
+      throw new Error('Could not get OffscreenCanvas context');
+    }
+
+    ctx.drawImage(bitmap, 0, 0);
+    
+    // 3. Convert to a small JPEG blob
+    const thumbnailBlob = await canvas.convertToBlob({
+      type: 'image/jpeg',
+      quality: 0.7
+    });
+
+    // 4. Release bitmap memory
+    bitmap.close();
+
+    // 5. Send result back
+    self.postMessage({ id, blob: thumbnailBlob });
+  } catch (err: any) {
+    self.postMessage({ id, error: err.message });
+  }
+};