feat: add NixOS support with flake.nix and installer detection

[SolAstrius]

Summary

• Add flake.nix with dev shell providing uv, Node.js 22, git, and native libs (openssl, sqlite, libffi) for Python wheel compilation
• Add .envrc for direnv integration (use flake)
• Detect NixOS in install.sh and skip system package installs — guide users to nix develop instead
• Python is managed by uv, not Nix (UV_PYTHON_PREFERENCE=only-managed)

Test plan

• nix develop enters shell with uv, node, git available
• uv venv && uv pip install -e . succeeds (native deps compile against nix-provided libs)
• cd web && npm ci && npm run build succeeds
• install.sh on NixOS: detects distro, skips system installs, prints nix develop guidance if tools are missing