Save button tooltip assumes Mac keyboard

Currently, the tooltip for the save button is hardcoded to use the Mac Command key icon (`&8984`), regardless of the user's environment. A better implementation would be to use the browser's user agent to determine the user's OS (using something like [`ua-parser-js`](https://www.npmjs.com/package/ua-parser-js)) and display an appropriate tooltip (Cmd-Enter for Mac OS and iOS, Ctrl-Enter for others).

Parsing OS information from the user agent isn't a perfect solution, but it's the best solution that will work across the vast majority of browser configurations. There is no way to determine if a user agent is accurate for the actual environment, so it will just have to be trusted.

Locate the save button tooltip and inspect how its keyboard shortcut is selected. Review ua-parser-js OS detection, then verify that Mac OS and iOS show Cmd-Enter while other environments show Ctrl-Enter; the issue is done when the tooltip adapts to the detected environment.

Written by the indexing model from the issue text.