Support Double Option as a conversion hotkey

[savinmikhail]

Summary

• generalize double-tap detection from Shift-only to Shift or Option
• add Double Option to the hotkey settings UI with backward-compatible preference migration
• keep Option+key shortcuts from triggering conversion
• add detector tests and update English/Russian documentation

Test plan

• GitHub Actions macOS test suite
• verify Double Shift and Double Option detection, timeout, and cancellation edge cases

[savinmikhail]

Local validation completed on Apple Silicon: all production Swift sources pass type-checking against the macOS 15.4 SDK, and the arm64 application binary compiles and links successfully. The full XCTest workflow is currently awaiting the repository owner approval required for first-time fork contributions.