docs(user): reader audio section + kobo guide refresh to platform reality

reader-guide.md gains the audiobook/episode player section (seek,
prev/next episode, Bookmarks panel, Chapters list, Player settings with
autoplay/speed/resume-rewind overrides, Shortcuts dialog) — the player
existed with zero user docs.

kobo-setup.md drops the stale 'not available yet, actively being
developed' framing: the server side of native Kobo sync is implemented
and documented (sync/initialization endpoints, bookmark + markup sync,
shelf mappings, KEPUB sidecar config), so the guide now states exactly
that, keeps stock-firmware rollout as the honest remaining piece, and
keeps KOReader as the supported today-path.

ui-controls.md: Player settings / Shortcuts & interactions / Chapters /
Bookmarks entries (templ-entity fragments for ampersand labels).

docs/user/reader-guide.md

@@ -2,7 +2,8 @@
 
 Bookhoard's built-in web reader works in any modern browser — desktop or
 mobile — and keeps your position, highlights, bookmarks, and notes in sync
-with the Android app and KOReader.
+with the Android app and KOReader. Audiobooks get their own player, covered
+at the end of this guide.
 
 ## Opening a Book
 
@@ -79,3 +80,25 @@ to jump to it.
 Position, highlights, bookmarks, and notes sync automatically through the
 server (see the [Sync Guide](sync-guide.md)). Close the tab whenever you
 like — the reader resumes exactly where you left off on any device.
+
+## Listening to audiobooks and podcast episodes
+
+Books with audio files, and podcast episodes, open in the audio player
+instead of the page reader. It is one player for both:
+
+- The **seek bar** is your position; drag it, or use the **previous /
+  next episode** controls when you are playing a sequence
+- **Bookmarks** opens a panel of your markers for this title — tap one to
+  jump back
+- **Chapters** opens the chapter list (shows "currently chapter N of M");
+  audiobooks with embedded chapters and podcast shows with chapter marks
+  both land here
+- **Player settings** holds the per-player preferences: **Autoplay on
+  open**, **Default playback speed**, and **Resume rewind** (a few seconds
+  back when you resume — the defaults come from your
+  [profile](profile-guide.md) and this panel overrides them per player)
+- The **Shortcuts & interactions** panel (the **?** control) lists every
+  keyboard shortcut
+
+Progress, bookmarks, and your speed carry across devices through the same
+sync as everything else.