fix: strip UTF-8 BOM before parsing config.json (#478)

getDefaultMode() parsed the config with JSON.parse and no BOM strip, so a
config.json saved with a UTF-8 BOM (common on Windows) failed to parse and the
user's defaultMode was silently ignored. Strip the BOM first, matching the
existing /^\uFEFF/ pattern in ponytail-activate.js, check-versions.js, and
ponytail-mode-tracker.js.

Closes #375

Co-authored-by: nanaubusiness <195150264+nanaubusiness@users.noreply.github.com>