Treat fast-forward beyond episode end as playback completion (#8592)

### Description

When fast-forwarding reaches or crosses the known end of an episode,
treat it as logical playback completion rather than seeking past the
known end and relying on Media3 to report playback completion.

This routes the boundary through the existing `handlePlaybackEnded()`
flow, preserving the established end-of-playback behavior for continuous
playback, queue progression, playback history, database updates, and
episode-based sleep timers.

Closes #8591

### Checklist
- [x] I have read the contribution guidelines:
https://github.com/AntennaPod/AntennaPod/blob/develop/CONTRIBUTING.md#submit-a-pull-request
- [x] I have performed a self-review of my code, going through my
changes line by line and carefully considering why this line change is
necessary
- [x] I have run the automated code checks using `./gradlew checkstyle
lint`
- [x] My code follows the style guidelines of the AntennaPod project:
https://antennapod.org/contribute/develop/app/code-style
- [x] I have mentioned the corresponding issue and the relevant keyword
(e.g., "Closes: #xy") in the description (see
https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
- [ ] If it is a core feature, I have added automated tests

playback/service/src/main/java/de/danoeh/antennapod/playback/service/Media3PlaybackService.java

@@ -164,7 +164,15 @@ public class Media3PlaybackService extends MediaLibraryService {
 
             @Override
             public void seekForward() {
-                seekTo(Math.min(getDuration(), getCurrentPosition() + UserPreferences.getFastForwardSecs() * 1000L));
+                long duration = getDuration();
+                long target = getCurrentPosition() + UserPreferences.getFastForwardSecs() * 1000L;
+
+                if (duration > 0 && target >= duration) {
+                    handlePlaybackEnded();
+                    return;
+                }
+
+                seekTo(target);
             }
 
             @Override