Summary:
- Enable narrow-screen layout for home page
- Clean up margins/spacing on small-screen home page
- Use "<768" as small-screen condition rather than "<=768".
- Include meta-viewport tag conditionally, off by default.
- Include "Toggle Mobile Mode" option in AccountMenu to toggle it on.
- In a test, add an after() clause to restore window size even when test fails
Test Plan: Only tested manually on iPhone (Safari & FF).
Reviewers: paulfitz
Reviewed By: paulfitz
Subscribers: cyprien
Differential Revision: https://phab.getgrist.com/D2708
