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/components/playerBar/PlayerVolume.tsx

@@ -40,7 +40,8 @@ export function PlayerVolume({
             setVolume(0);
           }
         }}
-        aria-label={t('player.volume')}
+        aria-label={volume === 0 ? t('player.unmute') : t('player.mute')}
+        data-tooltip={volume === 0 ? t('player.unmute') : t('player.mute')}
         style={{ color: 'var(--text-muted)', flexShrink: 0 }}
       >
         {volume === 0 ? <VolumeX size={16} /> : <Volume2 size={16} />}