feat: add SetWillCloseHook to WindowManager

[brennodewinter]

Summary

Adds WindowWillCloseHook to WindowManager, following the exact same pattern as the existing WindowWillShowHook and WindowWillHideHook. This allows FFI consumers (notably Flutter apps via nativeapi-flutter) to intercept window close and decide whether to proceed — needed for "unsaved changes" dialogs before closing.

Changes per layer

• window_manager.h: WindowWillCloseHook typedef, SetWillCloseHook, HasWillCloseHook, HandleWillClose, CallOriginalClose declarations.
• macOS: swizzles performClose: on NSWindow (same pattern as makeKeyAndOrderFront: and orderOut:). CallOriginalClose calls the swizzled original.
• Linux: adds a delete-event emission hook alongside show/hide. CallOriginalClose destroys the toplevel GtkWidget.
• Windows: stores the hook; CallOriginalClose posts WM_CLOSE. Full WM_CLOSE interception (WH_CBT or window subclassing) is left as a follow-up — noted with a ponytail: comment naming the ceiling and upgrade path.
• C API: native_window_manager_set_will_close_hook, has_will_close_hook, handle_will_close, call_original_close.
• Tests: window_manager_hook_test — verifies set/clear, dispatch with correct ID, no-hook safety, and coexistence with show/hide hooks.

How the hook works

The pattern is identical to will-show/will-hide:

1. Consumer calls SetWillCloseHook(callback) — platform installs swizzle/emission hook.
2. When the user closes a window, the swizzled method calls HandleWillClose(id) and returns without calling the original.
3. The hook decides: call CallOriginalClose(id) to proceed, or do nothing to prevent close.

Test plan

• window_manager_hook_test — 4 tests, all pass on macOS
• Full test suite: 6/6 tests pass on macOS
• Build verified on macOS (CMake + make, 0 errors)
• Build verified on Windows (CI)
• Build verified on Linux (CI)

Use case

OciDeck (a LibreKAT presentation tool) needs setPreventClose to show an "unsaved changes" dialog before the window closes. The current workaround uses window_manager's setPreventClose, but OciDeck is migrating to nativeapi. This hook is the missing piece.