improvement(desktop): bring agent browser menus, keys and fullscreen in line with Chrome

- Page-menu Back/Forward/Reload go through the user-navigation path, so the leave-site prompt and load-error handling apply
- Page menu gains spelling suggestions, Undo/Redo/Cut/Paste and Match Style/Select All, and image items (open, save, copy, copy address); Open Link in New Tab sends the referrer
- Spellcheck on in the agent browser on macOS
- Ctrl+wheel zoom steps the same zoom ladder as the menu
- Chrome keys: Alt+Left/Right, Cmd+Opt+Left/Right, Cmd+Shift+[ ], Ctrl+PgUp/PgDn, Esc stops loading, mouse back/forward buttons
- Page fullscreen fills the browser panel without resizing the Sim window

Author: waleedlatif1