Bring the second-window (beamer) presenter mode to all desktop platforms,
not just macOS:
- Implement the native window_coverScreen / window_close methods for the
vendored desktop_multi_window plugin on Windows (borderless popup over
the presentation monitor) and Linux.
- Register the app's plugins for sub-windows in the Windows and Linux
runners, so video/image rendering works in the audience window there too.
- Gate dual-screen mode through a testable shouldUseDualScreen() helper
(any desktop platform with >= 2 displays) and cover it with tests.
flutter analyze is clean and all presenter tests pass. Runtime two-screen
behaviour still needs verification on real hardware.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
ffcda70966