Build out assembly video activity

[jaellys]

Current implementation of assembly video: tap build button on main UI --> launch full-screen video

Few issues here:

• Experience is jarring: deep dive into ~20 minute technical video with no preface/orientation
• No text version of assembly walkthrough (accessibility issue)
• ~20 minute video without markers or assembly steps clearly indicated (should either cut into chapters or provide markers)

I think the video should live within an activity, providing an option for text instructions (could be a toggle, could be placed beneath the video—perhaps implementation would differ on tablet v. phones). We should either allow users to bookmark progress in the assembly video, or—if using chapters—track completed steps and keep that data in SharedPreferences.

What do y'all think?