codify "platform" into resolution logic, server-fn/fullstack overhaul (#4596)

overhaul dioxus fullstack using a new implementation of server_fn

This PR is a complete rewrite of dioxus fullstack designed to improve compatibility with axum and to fix a number of compilation errors based on feature flag unification plumbing through macros.

examples/assets/file_upload.css

@@ -8,7 +8,7 @@ body {
     min-height: 100vh;
 }
 
-#drop-zone {
+.drop-zone {
     border: 2px dashed #ccc;
     border-radius: 3px;
     padding: 20px;