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.
