Adds preliminary support for playing on mobile web

[willroberts]

Summary

This has already been deployed to https://staging.duelyst.org as a one-off. There are still a few remaining issues (see #220 and #221), but this makes things playable!

Closes #109.

App changes (app/, etc.):

• Changes viewport width from 800 to device-width to enable automatic scaling
• Reduces initial viewport scale from 1.0 to 0.1 (the minimum) which allows devices to scale up to their resolution
• Adds support for hiding the status/navigation bar in Google Chrome "Add to Home Screen" apps
• Updates docs

Testing

Have you have tested your changes in the following scenarios?
Feel free to check off scenarios which don't apply.

• Starting backend services locally with docker compose up succeeds.
• I am able to create a new user and log in locally.
• I am able to complete a practice game locally.
• I am able to complete a purchase of Orbs, etc.