web: bump font sizes and improve UI breakpoints (core #4728)

before = merge base f70c71c, after = pr head d716385. headless chromium, 3x dpi, light theme, logged out, prod data. desktop 1400x900, mobile 390x844. drag the slider (left = before, right = after). lightbox: full size, esc to close, arrow keys to switch pages, 1 = slider, 2 = side by side.

desktop / landing

desktop / login

desktop / repo

desktop / issues

desktop / pull

desktop / commits

mobile / landing

mobile / login

mobile / repo

mobile / issues

mobile / pull

mobile / commits