web: support a wider maximum width on all pages

right now, the web ui feels a little crammed with a maximum width of `800px`. i think we would benefit by making it wider (when the viewport allows us to, of course). a max-width of around `~1200-1400` feels right (i have two displays, only one of which is active at a given time: `3:2 2256x1506` and `16:9 2560x1440`).