Sundry macOS App and Web UI Improvements #703

Each of these various interface improvements and fixes was small enough that I did not think they merited an Issue and design discussion. However, in aggregate, they may need the attention of an Issue like this.

• PDF attachments: The macOS app and the web chat could attach images but not PDFs, even if the model could process them. Both can now attach PDFs. Attach PDFs in the macOS and web chats #686
• Local model files: Model files on this Mac could not be given a model name, a vision projector, or a draft in the app. They can now, per model, under Advanced › Model Files. Let the Mac app pick a local model's projector and draft #687
• Deleting chats: Chats could only be deleted one at a time. The app can now delete selected chats or all chats, and the web chat can delete all chats; each asks for confirmation first. Delete all chats function in app and web, and delete selected in the app #697
• Quitting: Quit stopped a running model without confirmation. It now prompts the user if a model is running, a download is active or a terminal program runs. Ask before quitting while a model, download or terminal program runs #698
• Menu bar menu: Pressing and holding the menu bar icon, dragging to a row and releasing did not choose it, as it does in any Mac menu. The menu could stop a model but not start one. It is now a native menu with a Start item. Use a native menu bar menu and add a Start item #699
• Dock icon: The app stayed in the Dock after its last window closed. It is now in the Dock only while a window is open, and Open Server and Open Chat in the menu bar menu bring the window and the Dock icon back. Use a native menu bar menu and add a Start item #699

All fixes have been rebased on top of 1.6.0 (9124ff2) and have been re-tested.