Navigating back can exit Speak Mode without PIN

[juliatuttle]

On (at least) the web and Android apps, using the back button, keyboard shortcut, or gesture can navigate out of Speak Mode (without requiring the PIN, if one is set) if you just re-entered Speak Mode.

My expectation when I navigate back in Speak Mode is generally that I will go back one board. For example:

1. I'm on Quick Core 112's home board, browsing around to learn what words are where. Generally, after entering a subboard, I can use the back gesture or shortcut to exit it, and it's closer than the back button by the vocalization box, so I do.
2. I press "help" to see what's in there, opening the help subboard.
3. I decide to [change the icon for the autism button from a puzzle piece heart to the neurodiversity infinity symbol]. I exit Speak Mode, edit the board, and re-enter Speak Mode.
4. I'm still on the help subboard, finish looking around, and decide I want to go back to the home board, so I use the back gesture.

At this point, I expect I'll see the home board, but instead I end up back out of Speak Mode. If that's infeasible (maintaining your own back stack is a pain in the butt), I'd settle for blocking the navigation back (like you do when I'm editing a board), showing a toast explaining how to exit Speak Mode, and maybe going to the home board -- it'd be less disruptive than the current experience.