feat: add thumbnail extraction and timeline display

Wire ffmpeg frame extraction into post-download processing and render
adaptive thumbnail strips on the timeline via the asset protocol.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lib/bindings/mediaAnalysis.ts

@@ -20,3 +20,13 @@ export async function extractWaveform(
 ): Promise<number[]> {
   return invoke<number[]>('extract_waveform', { filePath, sampleCount });
 }
+
+export async function extractThumbnails(
+  filePath: string,
+  duration: number
+): Promise<ThumbnailSpritesheet[]> {
+  return invoke<ThumbnailSpritesheet[]>('extract_thumbnails', {
+    filePath,
+    duration,
+  });
+}