Support a FlashCard before showing the notes that comprise a chord

My much anticipated feature: first prompt the user for a name of a chord, then
show the user that chord.

Cascading changes:
I changed the "Tap to practice" overlayButton's opacity from 30% to 100% because
pausing when showFlashCard is True causes the two piece

TIL:
You can batch Elm Subscriptions using the Sub.batch function.

What I haven't learned yet:
How to best handle rotating screens for mobile devices (i.e. portrait
vs. landscape modes). In time...

What's left?
- Support sound
- Support a fine-tune section of the preferences
- Support tablet and web browser variants
- Ask users for the "I chord" instead of asking "C major Root position"
- More styling (of course)

website/sandbox/learnpianochords/src/UI.elm

@@ -132,7 +132,7 @@ overlayButton { label, handleClick, isVisible } =
             , "top-0"
             , "left-0"
             , "block"
-            , "z-30"
+            , "z-40"
             , "w-screen"
             , "h-screen"
             , Tailwind.if_ isVisible "opacity-100" "opacity-0"
@@ -140,7 +140,7 @@ overlayButton { label, handleClick, isVisible } =
     in
     button
         [ classes |> Tailwind.use |> class
-        , style "background-color" "rgba(0,0,0,0.30)"
+        , style "background-color" "rgba(0,0,0,1.0)"
         , onClick handleClick
         ]
         [ h1