fix(macos): anchor popover to clicked display

[60dage]

Closes #5

Summary

• capture the macOS status-item click in native AppKit coordinates
• select the containing NSScreen and position the NSWindow against its visibleFrame
• remember the last native screen so a click on another display moves an open popover there, while a same-display click still toggles it closed
• retain the existing Positioner path on non-macOS platforms

Why

Tauri's physical monitor coordinates can be inconsistent on vertically arranged mixed-DPI macOS displays. Combining those values with AppKit window coordinates can select the wrong screen or place the popover outside every visible frame.

The click's NSEvent::mouseLocation() is already in the same coordinate space as NSScreen and NSWindow, so keeping the full placement path in AppKit coordinates avoids the scale conversion ambiguity.

Related upstream reports:

• [bug] [macos] physical position of windows and monitors reported incorrectly tauri-apps/tauri#7890
• [positioner] TrayCenter not working on extended monitor when using multiple monitors [macOS] tauri-apps/plugins-workspace#724

Testing

• cargo check --manifest-path src-tauri/Cargo.toml
• cargo test --manifest-path src-tauri/Cargo.toml — 95 passed
• npm test — 8 passed
• npm run build
• manual verification with a 1x external display arranged above a 2x Retina display:
  • lower → upper → lower → upper clicks
  • popover appeared under the clicked menu bar icon on both displays
  • full webview content remained visible after the final Vite reload

The reproduction setup is documented in #5; before/after screenshots will be attached there.