fix: clear thumbnail cache and temp dirs on new session

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

src/lib/stores/videoSession.svelte.ts

@@ -2,10 +2,12 @@ import type { VideoMetadata } from '$lib/bindings/video';
 import { startDownload } from '$lib/bindings/video';
 import type { ThumbnailSpritesheet } from '$lib/bindings/mediaAnalysis';
 import {
+  cleanupThumbnails,
   extractKeyframes,
   extractThumbnails,
   extractWaveform,
 } from '$lib/bindings/mediaAnalysis';
+import { clearThumbnailCache } from '$lib/timeline/thumbnailRenderer';
 
 export type SessionStatus = 'idle' | 'resolving' | 'ready' | 'error';
 export type DownloadStatus = 'idle' | 'downloading' | 'complete' | 'failed';
@@ -112,6 +114,10 @@ async function triggerPostDownloadProcessing(filePath: string) {
 export async function beginDownload() {
   if (!session.url) return;
 
+  const oldThumbnailDirs = [
+    ...new Set(session.thumbnailSpritesheets.map((sheet) => sheet.filePath)),
+  ];
+  clearThumbnailCache();
   session.downloadStatus = 'downloading';
   session.downloadProgress = 0;
   session.localFilePath = null;
@@ -119,6 +125,12 @@ export async function beginDownload() {
   session.waveformPeaks = [];
   session.thumbnailSpritesheets = [];
 
+  if (oldThumbnailDirs.length > 0) {
+    void cleanupThumbnails(oldThumbnailDirs).catch((err) => {
+      console.error('Thumbnail temp dir cleanup failed:', err);
+    });
+  }
+
   try {
     await startDownload(
       session.url,