Prepare release 0.3.0

[pitill0]

Summary

Prepares FluxTuner 0.3.0.

Changes include:

• Bump package version to 0.3.0.
• Update CHANGELOG.md with the completed roadmap work.
• Remove the duplicated hardcoded package version from fluxtuner/__init__.py.
• Read the runtime package version from installed package metadata via importlib.metadata.
• Document security, testing, CI, documentation, Flatpak, typing, search service and TUI modularization improvements.

Validation

• ruff check .
• ruff format --check .
• python -m compileall fluxtuner tests
• python -m pytest
• python -m build
• pip-audit --local
• bandit -r fluxtuner -c pyproject.toml
• mypy fluxtuner/core/search_service.py fluxtuner/core/importers.py fluxtuner/core/stations.py fluxtuner/players/security.py
• python -m fluxtuner --version