feat: add server-only build

it's drastically faster to build,
mostly due to only including 1/10th of the dependencies

src/main.rs

@@ -1,5 +1,6 @@
 use std::env;
 
+#[cfg(feature = "app")]
 mod app;
 mod server;
 mod sysinfo_snapshot;
@@ -7,10 +8,12 @@ mod utils;
 
 fn main() {
     match env::args().nth(1).as_deref() {
-        Some("server") => server::run().unwrap(),
-
+        #[cfg(feature = "app")]
         None | Some("client") => app::run().unwrap(),
 
+        #[cfg_attr(feature = "app", allow(unreachable_patterns))]
+        None | Some("server") => server::run().unwrap(),
+
         Some(s) => panic!("invalid argument: {s}"),
     }
 }