[mob] Fix: Duration computation from playlist (#5939)

## Description

## Tests

mobile/lib/services/preview_video_store.dart

@@ -653,13 +653,21 @@ class PreviewVideoStore {
 
   int? parseDurationFromHLS(String playlist) {
     final lines = playlist.split("\n");
+    double totalDuration = 0.0;
     for (final line in lines) {
-      if (line.startsWith("#EXT-X-TARGETDURATION")) {
-        final duration = line.split(":").last.trim();
-        return int.tryParse(duration);
+      if (line.startsWith("#EXTINF:")) {
+        // Extract duration value (e.g., "#EXTINF:2.400000," → "2.400000")
+        final durationStr = line.substring(
+          8,
+          line.length - 1,
+        );
+        final duration = double.tryParse(durationStr);
+        if (duration != null) {
+          totalDuration += duration;
+        }
       }
     }
-    return null;
+    return totalDuration > 0 ? totalDuration.round() : null;
   }
 
   Future<(String, String)> _getPreviewUrl(EnteFile file) async {