chore: migrate dependency management from Poetry to uv

[t-uda]

Overview

Migrate development and dependency management for ellphi from Poetry (pyproject.toml, poetry.lock) to uv (uv.lock, PEP 621 deps) now that demo dependencies are separated and ellphi-demo uses uv.

Tasks

• Remove all demo-related dependency definitions from pyproject.toml (optional-dependencies and poetry group)
• Remove Poetry-specific groups except for build if necessary
• Stop maintaining poetry.lock; add uv.lock or the minimal required lockfile
• Update development instructions (README, CONTRIBUTING, etc.) to use uv pipelines/commands instead of Poetry
• Update CI workflows to use uv for install, test, and check
• Sanity check import and build (matplotlib remains mandatory for now)

No

• Demo handling is now delegated to the separate ellphi-demo repo (already uv-based)
• No backwards compatibility needed for Poetry

[t-uda]

Resolved by PR #132 (merged into ellphi-0.1.3): Poetry fully replaced by uv — demo dependency definitions removed (demo lives in ellphi-demo), poetry.lock → uv.lock, dev/docs tooling in PEP 735 [dependency-groups], all four CI workflows on astral-sh/setup-uv with uv lock --check, and README/CONTRIBUTING/AGENTS.md/docs rewritten to uv commands. The poetry-core build backend is retained per this issue's 'except for build if necessary' clause (it drives the C++ backend build).