chore: prepare 0.5.0 release

[douglasmonsky]

Summary

• bump package, plugin manifest, and runtime MCP pin to 0.5.0
• add 0.5.0 changelog entry for dashboard localization
• move dashboard language details lower in the README so screenshots and usage workflow stay prominent
• align release/readiness docs with the 0.5.0 public package checks

Local checks

• python -m ruff check .
• python -m mypy
• python -m pytest
• python -m pytest --cov=codex_usage_tracker --cov-report=term-missing
• python -m compileall src
• node --check dashboard JS assets
• python scripts/check_release.py
• git diff --check
• python -m build
• python -m twine check dist/*
• python scripts/check_release.py --dist
• python scripts/smoke_installed_package.py

Release note

After this merges to main, publish 0.5.0 to TestPyPI first, smoke it, then publish to PyPI through the Trusted Publishing workflow.