Fix Rust warnings (#4872)

# Description of Changes
Fix warnings in Rust code on Mac. They were all being caused by file
handling logic which is now built into Tauri, so I've just been able to
remove all of the Mac specific file handling code.

I've also set warnings to be treated as errors because it'll be really
easy to accidentally introudce warnings on individual platforms which
I'm not developing, and I'd like to know about them so we can fix it
before getting dodgy code.

frontend/src-tauri/src/commands/files.rs

@@ -5,6 +5,7 @@ use std::sync::Mutex;
 static OPENED_FILE: Mutex<Option<String>> = Mutex::new(None);
 
 // Set the opened file path (called by macOS file open events)
+#[cfg(target_os = "macos")]
 pub fn set_opened_file(file_path: String) {
     let mut opened_file = OPENED_FILE.lock().unwrap();
     *opened_file = Some(file_path.clone());

frontend/src-tauri/src/commands/mod.rs

@@ -4,4 +4,6 @@ pub mod files;
 
 pub use backend::{start_backend, cleanup_backend};
 pub use health::check_backend_health;
-pub use files::{get_opened_file, clear_opened_file, set_opened_file};
+pub use files::{get_opened_file, clear_opened_file};
+#[cfg(target_os = "macos")]
+pub use files::set_opened_file;