Make videos better size on mobile

I'm not sure why here

https://github.com/FOSDEM/website/blob/245ba215013287496a09b7170f31a98c77312f1c/layouts/schedule/event.html#L62 

it's 75% instead of full width?

currently:

![Image](https://github.com/user-attachments/assets/d8da3fdc-41f1-4baa-ad98-098437a4ae83)

full width (100%):

![Image](https://github.com/user-attachments/assets/55c3cdd9-84ea-4aa8-b20b-671aebdcb7f6)