Podcast player: say why it cannot speak, with a way out

"The podcasts don't even work on my Linux setup on Firefox." Firefox on
Linux speaks through speech-dispatcher; without it speechSynthesis is present
but empty and silent - getVoices() is empty, speak() does nothing, no events
- so the player looked broken.

- pressing play with zero installed voices (or a start that never arrives
  within 1.7s) raises a banner naming the cause, with the Linux fix
  (sudo apt install speech-dispatcher) and the iOS one where it applies;
- the banner offers "Use the natural voice" when a provider key exists (it
  switches the source, keeps playing and remembers it), "Add a provider key"
  when not (opens Settings) and a link into the picker;
- natural-voice errors (refused autoplay, provider refusal) show in that same
  banner instead of hiding inside the picker.

podcast-test fakes a present-but-empty speechSynthesis and pins the banner,
the Linux line and the one-tap switch. 48 checks. Version 0.7.8, SW v100.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "freezmo",
-  "version": "0.7.7",
+  "version": "0.7.8",
   "private": true,
   "license": "GPL-3.0-only",
   "scripts": {