Switch from Poetry to uv for dependency management

[JuliusWelzel]

• Convert pyproject.toml from [tool.poetry] to PEP 621 [project] layout
• Replace Poetry specifiers with standard version ranges
• Move dev dependencies to PEP 735 [dependency-groups]; pytest is now dev-only rather than a runtime dependency
• Convert the docs extra to [project.optional-dependencies]
• Switch build backend from poetry-core to hatchling
• Replace poetry.lock with uv.lock
• Update CI (test-and-lint) to use astral-sh/setup-uv + uv sync/uv run
• Update the contributing guide to document uv instead of Poetry

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.79%. Comparing base (4c6b4e2) to head (8a4af48).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
- Coverage   87.43%   86.79%   -0.64%     
==========================================
  Files          23       23              
  Lines        2865     2863       -2     
==========================================
- Hits         2505     2485      -20     
- Misses        360      378      +18     

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