fix: clear thumbnail cache and temp dirs on new session

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

src/lib/timeline/thumbnailRenderer.ts

@@ -4,6 +4,10 @@ import { timeToX, type TimelineState } from './renderer';
 
 const imageCache = new Map<string, HTMLImageElement>();
 
+export function clearThumbnailCache(): void {
+  imageCache.clear();
+}
+
 const TARGET_SPACING_PX = 100;
 
 function getImagePath(dir: string, frameIndex: number): string {