Make Focus() / Blur() reliable and let Blur() return focus to the previous app

Start by locating the platform implementations of Window::Focus(), Blur(), and IsFocused(), then inspect the Windows, Linux, and macOS focus behavior described in the issue. Done means focus changes succeed reliably across those platforms and Blur() returns focus to the previously active window or app.

Window::Focus() and Blur() follow the platforms' simplest calls and fail in common cases:

• Windows: Focus() is SetForegroundWindow + SetFocus, which the foreground lock refuses when the app isn't in the foreground (only the taskbar flashes). Consider the AttachThreadInput / AllowSetForegroundWindow approaches, and check IsFocused() after focus changes.
• Linux: gtk_window_present() without a timestamp is subject to focus-stealing prevention; use gtk_window_present_with_time() with the last event time.
• Blur() should hand focus back to the previously active window or app. On macOS it only does orderBack:, and on Windows SetFocus(nullptr), which leaves the app active. Typical use: a launcher/search bar that hides and pastes into the previous app. Related: libnativeapi/nativeapi#59 (app-level hide on macOS).

Background:

• leanflutter/window_manager#364: focus() not always working, isFocused() always false (Windows)
• leanflutter/window_manager#218: focus() doesn't focus the window on Ubuntu
• leanflutter/window_manager#387: how to unfocus the window and bring back the previous one
• leanflutter/window_manager#352: focus the previous window when hiding