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.lock

@@ -155,12 +155,6 @@ dependencies = [
  "wyz",
 ]
 
-[[package]]
-name = "block"
-version = "0.1.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0d8c1fef690941d3e7788d328517591fecc684c084084702d6ff1641e993699a"
-
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -420,36 +414,6 @@ dependencies = [
  "windows-link 0.2.1",
 ]
 
-[[package]]
-name = "cocoa"
-version = "0.24.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f425db7937052c684daec3bd6375c8abe2d146dca4b8b143d6db777c39138f3a"
-dependencies = [
- "bitflags 1.3.2",
- "block",
- "cocoa-foundation",
- "core-foundation 0.9.4",
- "core-graphics 0.22.3",
- "foreign-types 0.3.2",
- "libc",
- "objc",
-]
-
-[[package]]
-name = "cocoa-foundation"
-version = "0.1.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8c6234cbb2e4c785b456c0644748b1ac416dd045799740356f8363dfe00c93f7"
-dependencies = [
- "bitflags 1.3.2",
- "block",
- "core-foundation 0.9.4",
- "core-graphics-types 0.1.3",
- "libc",
- "objc",
-]
-
 [[package]]
 name = "combine"
 version = "4.6.7"
@@ -502,19 +466,6 @@ version = "0.8.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
 
-[[package]]
-name = "core-graphics"
-version = "0.22.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2581bbab3b8ffc6fcbd550bf46c355135d16e9ff2a6ea032ad6b9bf1d7efe4fb"
-dependencies = [
- "bitflags 1.3.2",
- "core-foundation 0.9.4",
- "core-graphics-types 0.1.3",
- "foreign-types 0.3.2",
- "libc",
-]
-
 [[package]]
 name = "core-graphics"
 version = "0.24.0"
@@ -523,22 +474,11 @@ checksum = "fa95a34622365fa5bbf40b20b75dba8dfa8c94c734aea8ac9a5ca38af14316f1"
 dependencies = [
  "bitflags 2.10.0",
  "core-foundation 0.10.1",
- "core-graphics-types 0.2.0",
+ "core-graphics-types",
  "foreign-types 0.5.0",
  "libc",
 ]
 
-[[package]]
-name = "core-graphics-types"
-version = "0.1.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "45390e6114f68f718cc7a830514a96f903cccd70d02a8f6d9f643ac4ba45afaf"
-dependencies = [
- "bitflags 1.3.2",
- "core-foundation 0.9.4",
- "libc",
-]
-
 [[package]]
 name = "core-graphics-types"
 version = "0.2.0"
@@ -1992,15 +1932,6 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
-[[package]]
-name = "malloc_buf"
-version = "0.0.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "62bb907fe88d54d8d9ce32a3cceab4218ed2f6b7d35617cafe9adf84e43919cb"
-dependencies = [
- "libc",
-]
-
 [[package]]
 name = "markup5ever"
 version = "0.14.1"
@@ -2200,15 +2131,6 @@ dependencies = [
  "libc",
 ]
 
-[[package]]
-name = "objc"
-version = "0.2.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "915b1b472bc21c53464d6c8461c9d3af805ba1ef837e1cac254428f4a77177b1"
-dependencies = [
- "malloc_buf",
-]
-
 [[package]]
 name = "objc-sys"
 version = "0.3.5"
@@ -3689,7 +3611,7 @@ checksum = "18051cdd562e792cad055119e0cdb2cfc137e44e3987532e0f9659a77931bb08"
 dependencies = [
  "bytemuck",
  "cfg_aliases",
- "core-graphics 0.24.0",
+ "core-graphics",
  "foreign-types 0.5.0",
  "js-sys",
  "log",
@@ -3739,10 +3661,7 @@ checksum = "6ce2be8dc25455e1f91df71bfa12ad37d7af1092ae736f3a6cd0e37bc7810596"
 name = "stirling-pdf"
 version = "0.1.0"
 dependencies = [
- "cocoa",
  "log",
- "objc",
- "once_cell",
  "reqwest 0.11.27",
  "serde",
  "serde_json",
@@ -3887,7 +3806,7 @@ dependencies = [
  "bitflags 2.10.0",
  "block2 0.6.2",
  "core-foundation 0.10.1",
- "core-graphics 0.24.0",
+ "core-graphics",
  "crossbeam-channel",
  "dispatch",
  "dlopen2",