Experimental mpv-style (uosc) controls behind a Settings toggle

Thick timeline with hatched cache range, chapter diamonds and
chapter/time hover boxes; collapses to a thin progress line when
hidden. Flat icon row with track count badges, shader preset badge,
flat dark menus. Own show/hide state, live switch in window and
fullscreen. New buffered signal from demuxer-cache-time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -83,8 +83,9 @@ x to forget it. Ctrl+1 / Ctrl+2 switch between Join and Host.
 
 Profile, hardware decoding (auto, off or custom; the env var
 `SYNC_RELAY_HWDEC` overrides it; applies on the next join or Reload video),
-updates, playing the next Jellyfin episode automatically, video shaders
-(see below), and the keyboard shortcuts (read-only).
+"Experimental: mpv-style controls" (below), updates, playing the next
+Jellyfin episode automatically, video shaders (see below), and the
+keyboard shortcuts (read-only).
 Integrations links Jellyfin servers (below). About shows the version.
 
 Keys in a session: Space = ready / unready, Tab = side panel (people, log,
@@ -101,6 +102,16 @@ shows the time and the chapter title. The chapters button lists them
 Subtitles buttons (with the track count) list tracks as "Japanese · E-AC3
 2.0 · title"; subtitles have Off.
 
+Experimental mpv-style controls (Settings → Player, off by default,
+switches right away): video controls modeled on mpv's uosc theme. A thick
+timeline across the bottom shows the played part, the cached part ahead
+(hatched), chapter starts as diamonds and the time; hovering it shows the
+chapter title and the time there. With the controls hidden it stays as a
+thin line. Above it flat buttons: ≡ menu, Subtitles and Audio (with the
+track count), the shader badge (the active Anime4K mode, e.g. "A HQ"),
+ready, chapters, 1:30, previous / next episode, change video, people,
+volume and fullscreen. Keys and skip buttons work the same.
+
 Skipping: during the opening a "Skip OP" button floats in the corner (it
 jumps to the end of the opening), during the ending "Skip ED" (to the end
 of the ending only; what comes after is yours to skip). The opening and