fix(v3/macos): marshal application menu replacement to main thread

[leaanthony]

Description\n\nCalling after startup from a Wails custom-event listener crashes a macOS application:\n\n\n\nWails dispatches custom-event listeners on worker goroutines. calls the macOS implementation directly, and ultimately calls without main-thread dispatch.\n\n### Reproduction\n\n1. Create and set an initial application menu.\n2. Register a custom-event listener.\n3. From the listener, replace the menu with .\n4. Emit the event from the frontend after startup.\n\nThe application aborts in .\n\n### Expected behavior\n\nApplication-menu replacement is safe from Wails callbacks. The macOS backend should execute synchronously on the AppKit main thread, while avoiding a synchronous self-dispatch when already on that thread.\n\n### Acceptance criteria\n\n- no longer crashes when called from a background/custom-event callback on macOS.\n- Calls already on the main thread do not deadlock.\n- Existing application menu behavior remains unchanged on other platforms.\n- The macOS application package tests and build pass.

[coderabbitai]

🔗 Related PRs

#4435 - fix: prevent crash when closing window with pending async calls [merged]
#4947 - fix(v3): App Exposé shows “ghost” window after calling App.Window.Current() [merged]
#5165 - fix(v3/macos): apply setMenuItemChecked state synchronously [merged]
#5192 - fix(linux): fix race conditions and deadlocks in event handling [merged]
#5291 - fix(v3): prevent Menu.Update() segfault when app is not running [merged]
#5363 - fix(linux): use GUINT_TO_POINTER idiom for menu item IDs to fix systray disabling window menus [merged]
#5516 - fix(v3): prevent SIGSEGV enumerating screens on display change (macOS) [closed]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!