appview/login,signup: increase font size when <md to prevent focus zoom
safari on ios zooms on inputs if the font size is set to a value smaller
than 16px. setting the font size of <input> elements when the viewport
is smaller than `md` prevents that behavior
a broader fix would be to patch all inputs to behave that way. this
would require more manual work, this could be a point of focus when
transitioning to a better frontend framework.
resolves
[TAN-506](https://linear.app/tangled/issue/TAN-506/stop-safari-from-zooming-on-text-inputs)
Signed-off-by: eti <eti@eti.tf>