Add keyboard-controlled Agentation feedback in the desktop app #208

Add Agentation to the desktop app for precise visual feedback while dogfooding Ace. Open and close the toolbar through View → Toggle Annotations (Cmd+Shift+A), keep its collapsed launcher hidden, and preserve saved feedback when toggling. Feedback can be copied into an Ace channel.

Verify the shortcut in a real native app, including creation of an annotation, closing/reopening, and reopening the window. Keep the normal web app free of the toolbar.

Part of #5.