🐛 Fix Compose preview crash with rememberShareFileLauncher

[vinceglb]

Summary

• Defers FileKit.context access in Android settings classes until actually needed
• Makes authority parameter nullable with default null in FileKitShareSettings, FileKitOpenCameraSettings, and FileKitOpenFileSettings
• Adds getDefaultAuthority() helper to resolve the default authority when operations are executed

This prevents crashes in Android Studio Compose previews where the context is not initialized during composition.

Fixes #416

Test plan

• Verify Compose previews work with rememberShareFileLauncher()
• Verify Compose previews work with rememberCameraPickerLauncher()
• Verify sharing files still works at runtime on Android
• Verify camera picker still works at runtime on Android
• Verify opening files with default application still works at runtime on Android

🤖 Generated with Claude Code