detect mode and device

[amitu]

When using fastn in dynamic mode (running fastn serve on a server instead of using fastn build), we generate default HTML with assumptions about the device. If the assumption is incorrect we nuke the DOM after page load and recreate the page.

We can do better by better guessing the device by looking at user agent for the first page load scenario. For dark mode preferences we can use Sec-CH-Prefers-Color-Scheme.

We can further set cookie by detecting these properties on client side, and rely on the cookie to send the right HTML.