Add optional Vietnamese lunar calendar dates

[andynvt]

Summary

Adds an optional Vietnamese lunar calendar display to Itsycal.

Changes

• Add a “Show Lunar Calendar (Vietnamese)” appearance preference, disabled by default.
• Display lunar dates below Gregorian dates in the calendar grid.
• Include lunar dates in agenda date headers.
• Resize calendar cells automatically when lunar dates are enabled.
• Support lunar leap months using UTC+7 calculations.
• Update the calendar and agenda immediately when the preference changes.

Testing

• Built successfully with the Debug configuration using xcodebuild.
• Launched and verified the app on macOS.
• Verified lunar dates render in the calendar when enabled.

Screenshots