[x] Layout chat and new input to be more responsive to narrow widths
[ ] Improve target sizes for inputs (the + and some other buttons are too small to reliably tap)
[ ] Full-width layout for File and Options tabs on narrow screens
[ ] Figure out how to handle iOS default zoom.
iOS on default zooms in on the page and breaks the layout for inputs with fontSize < 16px. We can accept this, default to 16px font size for editor text on mobile (Adam needs to figure out how to do this), or disable all user zooming (bad for a11y)
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