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/Cargo.toml

@@ -10,6 +10,9 @@ rust-version = "1.77.2"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[lints.rust]
+warnings = "deny"
+
 [lib]
 name = "app_lib"
 crate-type = ["staticlib", "cdylib", "rlib"]
@@ -27,9 +30,3 @@ tauri-plugin-shell = "2.1.0"
 tauri-plugin-fs = "2.4.4"
 tokio = { version = "1.0", features = ["time"] }
 reqwest = { version = "0.11", features = ["json"] }
-
-# macOS-specific dependencies for native file opening
-[target.'cfg(target_os = "macos")'.dependencies]
-objc = "0.2"
-cocoa = "0.24"
-once_cell = "1.19"