Every play button for what is playing shows pause, and pauses it

Only the player bar's button changed; the files pane's, the row's and the
toolbar's kept showing play while it played. play() now pauses when asked to play
what is already playing, which makes each of them a toggle, and syncPlayButtons()
repaints them on play, pause and ended and whenever the list or reader is drawn.

Closes #34.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -48,6 +48,8 @@ The long form, with what was wrong before and how it was found, is in
   higher than a downloaded one's.
 - Images in posts from sites that refuse images to other sites' pages, such as Jeff Geerling's,
   now show: ipx asks for them without saying it is the page showing them.
+- While an episode plays, its play buttons in the files pane, its row and the toolbar show
+  pause, as the player bar's does, and pause it when pressed.
 - An item you open stays read. A list refresh that crossed with marking it read could put its
   unread dot back until the next refresh.
 - On the Unread tab, the item you were reading leaves the list as soon as you move to the next