fix(player): the Plex bars hold while hovered, volume is the native slider, tooltips no longer re-layer the row (E28)

- F22: the control bar faded out from under a mouse parked on it; the hide timer
  now waits while the pointer is on either band and re-arms on leave (test seen
  RED by disabling the hold).
- The hand-drawn volume bar broke DESIGN.md A6 (native sliders): it had no hover,
  focus, keyboard or screen-reader state. It is the Slider primitive now; a
  pointer drag blurs it so Space and the arrows keep driving the player.
- The control tooltips are composited from mount (will-change: opacity): a fade
  that promoted one on hover re-layered its neighbours under the player's
  scale(1.25), and the whole row shimmered.

frontend/src/i18n/locales/en/plex.json

@@ -171,6 +171,7 @@
     "prev": "Previous episode",
     "next": "Next episode",
     "mute": "Mute (m)",
+    "volume": "Volume",
     "fullscreen": "Fullscreen (f)",
     "download": "Download original file",
     "tracks": "Audio & subtitles",

frontend/src/i18n/locales/hu/plex.json

@@ -171,6 +171,7 @@
     "prev": "Előző rész",
     "next": "Következő rész",
     "mute": "Némítás (m)",
+    "volume": "Hangerő",
     "fullscreen": "Teljes képernyő (f)",
     "download": "Eredeti fájl letöltése",
     "tracks": "Audió és felirat",