First‐class support for the new Mail extensions on macOS 13+

[theZ3r0CooL]

Falls back to the old .mailbundle plugin on earlier releases

• `MailPluginManager.swift`
  – Added `supportsMailExtension` check (#available(macOS 13, *)).
  – Updated `isMailPluginInstalled` to immediately returns true on 13+, otherwise does the old bundle-UUID check.
  – `installMailPlugin()` is a no-op on 13+ (just opens Mail.app to let the user enable the extension) & still copies the bundle on older macOS.
  – `uninstallMailPlugin()` & `pluginDownload()` similarly skip bundle actions on 13+.

• `OpenHaystackSettingsView.swift`
  – The toggle label now switches based on OS version: “Use Apple Mail Plugin (≤12)” vs “Enable Mail Extension (≥13)”.

• `OpenHaystackMainView.swift`
  – The toolbar “Reload” button reads “Reload Extension” on 13+ (stays “Reload” on older macOS).
  – The popover text dynamically refers to the “mail extension” vs “mail plug-in” based on runtime availability.

• `OpenHaystackTests.swift`
  – `testPluginInstallation()` now skips itself on macOS 13+ with `try XCTSkip(...)` & only runs the bundle-copy assertions on older OS.