cmd/mbf: bring back pkgserver's favicon!

It existed in mae's #33, but ozy seems to have lost it during her
changes pre-merge, so just add it back again.

This favicon image was grabbed from mae:
8a38b614c6/cmd/pkgserver/ui/static/favicon.ico
That commit is the latest one of the salvaged original #33 history; see
#33 (comment).

cmd/mbf/internal/pkgserver/ui/ui_full.go

@@ -8,7 +8,7 @@ import (
 )
 
 //go:generate tsc
-//go:generate cp index.html style.css static
+//go:generate cp index.html style.css favicon.ico static
 //go:embed static
 var _static embed.FS