Fix SDL3 focus handling: restore mouse grab and cursor state on alt-tab/Win key

[Copilot]

On Windows with SDL3, alt-tabbing or pressing the Win key leaves the cursor invisible on the desktop and breaks camera rotation when clicking back into the app.

Root causes in SdlMouseInput:

• SDL_HideCursor() is never reversed on focus loss → cursor invisible everywhere outside the app
• SDL_SetWindowRelativeMouseMode + mouse grab are released by SDL3 on focus loss but never re-applied on focus regain → mouse delta input stops working

Fix: Handle SDL_EVENT_WINDOW_FOCUS_LOST and SDL_EVENT_WINDOW_FOCUS_GAINED in SdlMouseInput.onSDLEvent():

• Focus lost: explicitly release capture/grab/relative mode; call SDL_ShowCursor() if cursor was hidden; reset X11 warp state to avoid stale warp events
• Focus gained: call setCursorVisible(cursorVisible) to re-apply grab, relative mode, and cursor visibility as they were before focus was lost

if (type == SDL_EVENT_WINDOW_FOCUS_GAINED) {
    if (event.window().windowID() == context.getWindowId()) {
        setCursorVisible(cursorVisible);  // re-enables grab + relative mode
    }
    return;
}
if (type == SDL_EVENT_WINDOW_FOCUS_LOST) {
    if (event.window().windowID() == context.getWindowId()) {
        x11WarpGrabMode = false;
        ignoreNextX11WarpEvent = false;
        SDL_CaptureMouse(false);
        SDL_SetWindowMouseGrab(context.getWindowHandle(), false);
        SDL_SetWindowRelativeMouseMode(context.getWindowHandle(), false);
        if (!cursorVisible) SDL_ShowCursor();
    }
    return;
}