More fullstack fixes (#4737)

* allow `--server` only builds!

* rollback example

* capture `/:route` routes

* fix: properly implement file uploads in forms

* implement desktop form uploads

* Performant serialization of form bytes

* finish up file stuff

* cleanups

* add request headers to streaming context

* remove some unwraps

Cargo.lock

@@ -14089,6 +14089,7 @@ dependencies = [
  "js-sys",
  "log",
  "mime",
+ "mime_guess",
  "native-tls",
  "percent-encoding",
  "pin-project-lite",