fix(playerbar): add Mute / Unmute tooltip on the speaker icon (#752)

The speaker button in the player bar toggled volume to 0 (or back to
the pre-mute level) but exposed no `data-tooltip`, so hovering it
showed nothing — users couldn't tell what the icon did. Add a tooltip
that switches between "Mute" and "Unmute" with the current state, and
mirror the same string on `aria-label` for screen readers. New
`player.mute` / `player.unmute` keys land across all 9 locales.

src/locales/de/player.ts

@@ -39,6 +39,8 @@ export const player = {
   repeatOne: 'Einen',
   progress: 'Songfortschritt',
   volume: 'Lautstärke',
+  mute: 'Stummschalten',
+  unmute: 'Stumm aufheben',
   toggleQueue: 'Warteschlange umschalten',
   collapseQueueResize: 'Warteschlange einklappen, Breite ändern',
   moreOptions: 'Weitere Optionen',