feat(player): Add 3-finger tap gesture for video snapshots

Adds an optional 3-finger tap gesture to capture video snapshots during playback and save them to the Camera Roll.

Closes #386 (closed)

Changes

• Add saveSnapshotWithCompletion: method in VLCPlaybackService using saveVideoSnapshotAt:
• Add 3-finger tap gesture recognizer in VideoPlayerViewController
• Add settings toggle in Gestures section (disabled by default)
• Add localized strings for settings and user feedback alerts
• Use polling for file creation (50ms intervals, max 2s timeout)

Implementation

The snapshot is saved as a PNG at native video resolution, providing higher quality than a screen screenshot and without the VLC UI overlay.

Testing

1. Enable "3 finger tap to snapshot" in Settings → Playback Controls
2. Play any video
3. Tap with 3 fingers
4. Verify success alert appears and image is saved to Photos app