The embedded filesystem was in package main, making it inaccessible to
alternative main packages that need to serve the HTTP interface. Moving
the embed directive and web handlers to a dedicated web package allows
any package to import and use the web UI functionality.
This also improves separation of concerns by isolating static file
serving, template handling, and dev server proxying in a single package,
and simplifies the API by removing the staticContent parameter from
NewHandler.
Ref #512 and #15
