Fix: prevent thumbnail cache from being cleared on empty video lists

This commit addresses an issue where thumbnail images would "drop" (disappear/flicker) upon login or re-initialization of the video list.

Changes:
- Modified `js/ui/views/VideoListView.js` to add a guard clause in `cleanupThumbnailCache`.
- The function now returns early if the provided `videos` list is empty, preserving the existing `loadedThumbnails` cache during transient loading states.

Verification:
- Created a reproduction script (`reproduce_issue.mjs`) that confirmed the cache was being cleared on empty input.
- Verified the fix with the same script, ensuring the cache is preserved.
- Verified no regression in `npm run lint:hex` and `npm run lint:inline-styles`.

js/ui/views/VideoListView.js

@@ -1011,6 +1011,10 @@ export class VideoListView {
       return;
     }
 
+    if (!Array.isArray(videos) || videos.length === 0) {
+      return;
+    }
+
     const activeIds = new Set();
     videos.forEach((video) => {
       if (video && typeof video.id === "string" && video.id) {