Fix fullscreen macos26 grey border fullscreen

[therealmarv]

fixes #82 by handling the fullscreen window in a special way for macOS 26 Tahoe. I could not test it with older macOS but it should cause no problems (that needs testing).

Short Summary

• Fix grey border on fullscreen on macOS 26 by ensuring a fully opaque, black canvas at all times
• Image view now paints black before any image and is declared opaque for avoiding grey window borders
• Fullscreen uses constraint-based layout; windowed uses autoresizing; no mixing of layout systems (could not get the new fullscreen handling to work in window mode)
• Overhauled window config:
  • fullscreen is borderless, opaque, no shadow, hidden title
  • windowed is titled, opaque, with shadow
• Notch/safe-area handled in fullscreen by offsetting the top constraint
• Centralized screen configuration; avoid manually setting view frames when constraints are active