fix: validate stream_url and clear stale session on error

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

src-tauri/src/services/video_resolver.rs

@@ -60,6 +60,10 @@ pub fn resolve(url: &str, cookie_source: &CookieSource) -> Result<VideoMetadata,
         .trim()
         .to_string();
 
+    if stream_url.is_empty() {
+        return Err("yt-dlp returned an empty stream URL".to_string());
+    }
+
     Ok(VideoMetadata {
         url: url.to_string(),
         title: meta.title,