fix(config): Make `macos-custom-icon` null-terminated (#8999)

The config option `macos-custom-icon` wasn't working because, to pass
successfully through the C API to Swift, the string must be
null-terminated.

Fixes
https://discord.com/channels/1005603569187160125/1423192859112116224

src/config/Config.zig

@@ -2875,7 +2875,7 @@ keybind: Keybinds = .{},
 ///
 /// Note: This configuration is required when `macos-icon` is set to
 /// `custom`
-@"macos-custom-icon": ?[]const u8 = null,
+@"macos-custom-icon": ?[:0]const u8 = null,
 
 /// The material to use for the frame of the macOS app icon.
 ///