chore: add codespell pre-commit hook and fix typos (#1952)

- Add codespell v2.4.2 to .pre-commit-config.yaml with tomli dependency
- Add [tool.codespell] config to pyproject.toml: skip openapi_generated and *.ipynb, ignore-words-list for technical terms (nce, inpt, nd, wirth)
- Fix ~50 spelling errors across lightly/, tests/, docs/, examples/

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Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: Lionel Peer <lionel@lightly.ai>