Update Helium to 0.16.3.1 and add Cargo module

Bump Helium AppImage version and update its SHA‑256 hash.
Add a new home module that installs a .cargo/config.toml with a custom
registry and enables git‑fetch‑with‑cli.

modules/home/default.nix

@@ -17,6 +17,7 @@ _: {
     ./anki.nix
     ./bat.nix
     ./btop.nix
+    ./cargo.nix
     ./cava.nix
     ./easyeffects.nix
     ./eza.nix