fix: wrap NSView::new in unsafe block for objc2 compatibility

[welchjp]

Summary

NSView::new was made unsafe in objc2-app-kit 0.3.x as part of objc2's ongoing safety audit. This causes compilation errors when building with current dependency versions:

error[E0133]: call to unsafe function `NSView::new` is unsafe and requires unsafe function or block
  --> src/popover.rs:30:27
   |
30 |         let placeholder = NSView::new(mtm);
   |                           ^^^^^^^^^^^^^^^^ call to unsafe function

Fix

Wrapped the NSView::new call in an unsafe block with a safety comment. The call is safe because we have a valid MainThreadMarker, which guarantees we're on the main thread as required by AppKit.

Testing

• Tested with cargo check - compiles successfully
• Tested in a Tauri 2.x application - popover displays and functions correctly

[freethinkel]

Thank you for the PR! LGTM