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/developer/ui-controls.md

@@ -68,6 +68,27 @@ note: templ source stores the ampersand entity; the rendered label reads "Add &
 label: Download all episodes now
 where: Add Podcast Feed panel
 does: Grabs the feed's back catalog immediately on subscribe.
+
+label: Player settings
+where: audio player → settings control
+does: Opens the per-player preferences — Autoplay on open, Default playback speed, Resume rewind.
+also: title="Settings"
+
+label: Shortcuts & interactions
+where: audio player → the ? control
+does: Lists every keyboard shortcut the player supports.
+also: Shortcuts & interactions
+note: templ source stores the ampersand entity
+
+label: Chapters
+where: audio player → chapter-list control
+does: Opens the chapter list and shows the current chapter position.
+note: title attribute on the control
+
+label: Bookmarks
+where: audio player → bookmarks control
+does: Opens your markers for this title; tap one to jump back.
+note: title attribute on the control
 ```
 
 ## Collections