refactor(watch): single watched-progress predicate

lib/media/watch_progress.dart

@@ -0,0 +1,8 @@
+/// Single source of truth for the "progress crossed the watched threshold"
+/// decision. All comparison sites (online progress events, external-player
+/// returns, offline queueing) must route through this so edge-case handling
+/// (zero/unknown duration, exact-threshold) can't drift between paths.
+bool isWatchedProgress({required int positionMs, required int durationMs, required double threshold}) {
+  if (durationMs <= 0) return false;
+  return positionMs / durationMs >= threshold;
+}

lib/services/external_player_service.dart

@@ -7,6 +7,7 @@ import 'package:flutter/services.dart';
 
 import '../media/media_item.dart';
 import '../media/media_server_client.dart';
+import '../media/watch_progress.dart';
 import '../models/external_player_models.dart';
 import '../utils/app_logger.dart';
 import '../utils/snackbar_helper.dart';
@@ -207,7 +208,11 @@ class ExternalPlayerService {
       watchedThreshold: client.watchedThreshold,
     );
 
-    if (position.inMilliseconds / duration.inMilliseconds >= client.watchedThreshold) {
+    if (isWatchedProgress(
+      positionMs: position.inMilliseconds,
+      durationMs: duration.inMilliseconds,
+      threshold: client.watchedThreshold,
+    )) {
       try {
         // reportPlaybackStopped above marks the item played on backends that
         // support it (Jellyfin); markWatchedFromPlaybackStop then only emits the

lib/services/offline_watch_sync_service.dart

@@ -11,6 +11,7 @@ import '../media/media_item.dart';
 import '../media/media_kind.dart';
 import '../media/media_server_client.dart';
 import '../media/playback_report_metadata.dart';
+import '../media/watch_progress.dart';
 import '../utils/app_logger.dart';
 import '../utils/global_key_utils.dart';
 import 'offline_mode_source.dart';
@@ -275,9 +276,8 @@ class OfflineWatchSyncService extends ChangeNotifier {
 
   /// Check if an item should be considered watched based on progress percentage.
   bool isWatchedByProgress(int viewOffset, int duration, {ServerId? serverId}) {
-    if (duration == 0) return false;
     final threshold = serverId != null ? getWatchedThreshold(serverId) : 0.9;
-    return (viewOffset / duration) >= threshold;
+    return isWatchedProgress(positionMs: viewOffset, durationMs: duration, threshold: threshold);
   }
 
   /// Get the local watch status for a media item.

lib/utils/watch_state_notifier.dart

@@ -1,5 +1,6 @@
 import '../media/media_item.dart';
 import '../media/ids.dart';
+import '../media/watch_progress.dart';
 import 'app_logger.dart';
 import 'base_notifier.dart';
 import 'global_key_utils.dart';
@@ -130,7 +131,7 @@ class WatchStateNotifier extends BaseNotifier<WatchStateEvent> {
       appLogger.w('WatchStateNotifier: missing serverId for ${item.id}, skipping progress event');
       return;
     }
-    final isNowWatched = duration > 0 && (viewOffset / duration) >= watchedThreshold;
+    final isNowWatched = isWatchedProgress(positionMs: viewOffset, durationMs: duration, threshold: watchedThreshold);
 
     notify(
       WatchStateEvent(

test/media/watch_progress_test.dart

@@ -0,0 +1,27 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:plezy/media/watch_progress.dart';
+
+void main() {
+  group('isWatchedProgress', () {
+    test('false for zero or negative duration', () {
+      expect(isWatchedProgress(positionMs: 1000, durationMs: 0, threshold: 0.9), isFalse);
+      expect(isWatchedProgress(positionMs: 1000, durationMs: -1, threshold: 0.9), isFalse);
+    });
+
+    test('exact threshold counts as watched', () {
+      expect(isWatchedProgress(positionMs: 90, durationMs: 100, threshold: 0.9), isTrue);
+    });
+
+    test('just below threshold is not watched', () {
+      expect(isWatchedProgress(positionMs: 89, durationMs: 100, threshold: 0.9), isFalse);
+    });
+
+    test('position past duration is watched', () {
+      expect(isWatchedProgress(positionMs: 110, durationMs: 100, threshold: 0.9), isTrue);
+    });
+
+    test('zero position is not watched', () {
+      expect(isWatchedProgress(positionMs: 0, durationMs: 100, threshold: 0.9), isFalse);
+    });
+  });
+}