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/07-fullstack/ssr-only/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "ssr-only"
+edition = "2024"
+version.workspace = true
+publish = false
+
+[dependencies]
+dioxus = { workspace = true, features = ["fullstack", "router"] }
+
+[features]
+default = ["server"]
+server = ["dioxus/server"]