Fix presenter video playback in read-only presentation mode

[ErickWendel]

Summary

• Hide the read-only media placeholder in presenter view.
• Ensure presenter video elements render frames through Konva.
• Add unit coverage for video rendering and an end-to-end check that frames change when seeking.

Testing

• Added unit coverage in PresenterView.test.tsx.
• Added Playwright coverage for presenter video frame rendering and seeking.