fix: add wind speed unit preference (#703)

[Orinks]

Summary

• add a dedicated wind speed unit preference with Auto, mph, km/h, and m/s options
• route the chosen wind unit through current conditions, forecast, hourly, and tray formatting
• cover the new wind preference plumbing with targeted unit and settings tests

Testing

• .venv/bin/python -m pytest -q tests/test_unit_utils.py tests/test_presentation_formatters.py tests/test_round_values_setting.py tests/test_settings_dialog_tray_text.py tests/test_display_tab_unit_labels.py tests/test_settings_notification_toggles.py
• .venv/bin/ruff check src/accessiweather/app_lifecycle.py src/accessiweather/display/presentation/current_conditions.py src/accessiweather/display/presentation/forecast.py src/accessiweather/display/presentation/forecast_hourly.py src/accessiweather/display/presentation/measurement_formatters.py src/accessiweather/display/weather_presenter.py src/accessiweather/models/config_constants.py src/accessiweather/models/config_serialization.py src/accessiweather/models/config_settings.py src/accessiweather/models/config_validation.py src/accessiweather/taskbar_icon_updater.py src/accessiweather/ui/dialogs/settings_dialog_handlers.py src/accessiweather/ui/dialogs/settings_tabs/display.py src/accessiweather/units.py tests/test_unit_utils.py tests/test_settings_dialog_tray_text.py