Add discoverable rewind-10 / forward-10 controls to the Plex player control
bar, flanking play/pause (D5-A). New Skip10 glyph: an outline circular arrow
with a centred '10', deliberately distinct from the episode-level
SkipBack/SkipForward and from Restart's plain RotateCcw. Reuses seekTo; mirrors
the keyboard ←/→ (seekStep=10). i18n HU/EN aria-labels.
S4a-1 of the PlexPlayer decomposition. Move the module-level pure
helpers (prefs types/defaults, track-language resolution, subtitle
shadow CSS, wall-clock formatting, HLS_SUB_LAG, fmt) to
lib/plexPlayerPrefs.ts and the presentational building blocks
(Ctrl/Slider/Toggle/PanelHead) to plexPlayerControls.tsx. No hooks or
behaviour touched; PlexPlayer.tsx 1889->1661.
peter