build: add Nix packages and development environment

[0xsharkboy]

Summary

• add reproducible Nix packages and apps for the GUI and CLI on x86_64-linux and aarch64-linux
• provide a pinned development shell with Rust nightly, Node.js, WASM support, and contribution tooling
• install desktop integration and disable the self-updater for immutable Nix builds
• validate both packages with a native Linux CI matrix and document the Nix workflows

Testing

• nix flake check --all-systems --no-build
• nix build .#objdiff .#objdiff-cli --no-link --print-build-logs
• nix develop --command cargo test
• nix develop --command cargo +nightly clippy --all-targets --all-features --workspace -- -D warnings
• nix develop --command cargo +nightly fmt --all --check
• nix develop --command npm -C objdiff-wasm run build