Fix: Replace AsyncWorker with ThreadSafeFunction to prevent crashes

[brandonkboswell]

• Replaced CalendarAccessWorker, RemindersAccessWorker, and RemindersFetchWorker with thread-safe context structs
• Use ThreadSafeFunction instead of AsyncWorker for cross-thread callbacks
• Capture shared_ptr by value in lambdas to ensure promise lifetime
• Fixes segmentation fault when macOS permission dialog callback fires

This fixes crash at memory address 0x260 when users grant calendar permissions.

[brandonkboswell]

@dacay let me start by saying I'm not an Objective-C coder or a Swift coder of any kind. I found that EventKit-node was crashing after accepting permissions when I went to use it. I had Claude code fix that issue, and the code that's in this fork seems to continue to maintain all the functionality that existed in your version, but doesn't crash after receiving proper permissions on macOS 26.2 Tahoe.