fix: use filesystem-safe encoding for lock file paths

[Shubham-Padkonde]

Some Unicode configuration paths encode to Base64 containing /. open_with_lockfile() then treats that slash as a directory separator in the temporary lock path and raises FileNotFoundError before opening the configuration file.

Use URL-safe Base64 so the generated lock name stays a single filename. Add tests for Unicode paths, competing lock acquisition, reopening after release, and cleanup when the context raises, plus a changelog entry.

Validation: 17 utility/update-check tests pass, and changed-file Flake8 passes. The two new Unicode cases fail before the fix. A broader run encountered two Big5 charset-detection failures, both reproduced with unchanged upstream code; the full suite was interrupted during slow network tests, so I cannot claim a complete full-suite pass.