Nix build fixes:
- Override beamPackages.elixir to use 1.19.5 instead of default 1.18.4
- Add explicit buildPhase to compile dependencies before application
- Remove postInstall phase (Mix automatically includes priv/ in release)
- Set HEX_OFFLINE=false to allow dependency fetching in Nix sandbox
Docker image build now succeeds (~507 MB compressed).
Also remove .claude/settings.local.json from git tracking (keep local).
File remains in working directory but is now properly ignored.