Localize signal snapshot price source labels

[Pigbibi]

Summary

• add shared helpers for localized signal snapshot price source labels
• add shared quote-overlay yes/no/unknown localization
• cover Schwab and LongBridge source labels in unit tests

Verification

• PYTHONPATH=src .venv/bin/pytest tests/test_notification_localization.py -q
• PYTHONPATH=src .venv/bin/ruff check src/quant_platform_kit/common/notification_localization.py tests/test_notification_localization.py
• git diff --check