build: switch from devenv to nix flakes with uv2nix

- Replace devenv setup with pure nix flakes using uv2nix, pyproject-nix,
  pyproject-build-systems, and git-hooks.nix (adapted from hokusai)
- Add forgejo CI pipeline with lint/check/test and build jobs
- Wrap cv binary with LD_LIBRARY_PATH for weasyprint native dependencies
- Move basedpyright to dev dependency group, add pytest and ruff
- Fix package imports to use hatch source remapping (src/ layout)
- Update .envrc to use flake, expand .gitignore

BREAKING CHANGE: devenv is no longer used, run direnv allow to reload

main.py

@@ -2,7 +2,7 @@ from pathlib import Path
 
 import typer
 
-from src.cv_generator.generator import generate_pdf
+from cv_generator.generator import generate_pdf
 
 app = typer.Typer()