chore(deps): update flake.lock

Flake lock file updates:

• Updated input 'flake-parts':
    'github:hercules-ci/flake-parts/5635c32' (2025-12-11)
  → 'github:hercules-ci/flake-parts/a34fae9' (2025-12-15)
• Updated input 'flake-parts/nixpkgs-lib':
    'github:nix-community/nixpkgs.lib/719359f' (2025-10-29)
  → 'github:nix-community/nixpkgs.lib/2075416' (2025-12-14)
• Updated input 'nixpkgs':
    'github:nixos/nixpkgs/28bb483' (2025-12-13)
  → 'github:nixos/nixpkgs/b3aad46' (2025-12-20)
• Updated input 'nixpkgs-unstable':
    'github:nixos/nixpkgs/2fbfb1d' (2025-12-11)
  → 'github:nixos/nixpkgs/c6245e8' (2025-12-18)
• Updated input 'system-manager':
    'github:numtide/system-manager/a015a47' (2025-12-12)
  → 'github:numtide/system-manager/bd3e419' (2025-12-17)
• Updated input 'treefmt-nix':
    'github:numtide/treefmt-nix/5b4ee75' (2025-11-12)
  → 'github:numtide/treefmt-nix/42d96e7' (2025-12-17)

Author: github-actions[bot]