Audiobookshelf client: audiobooks and podcasts from the server people actually run

[cubman3134]

#139 gives us a local audiobook library. Audiobookshelf is the self-hosted server that audiobook listeners overwhelmingly run — and Symfonium treating it as a first-class source is a reminder that we have the local half and nothing for the networked one. Same consume-side pattern as #83 (Jellyfin), #146 (OPDS), and #193 (Subsonic).

Proposed behaviour — shaped like #193, since the problem is the same:

• Config: server URL + credentials (Audiobookshelf issues an API token on login), multiple servers supported, server-qualified item ids from day one per the Multiple Jellyfin servers, merged into one library #160/Subsonic client: play from a Navidrome, Airsonic, or Gonic library you already run #193 rule. Credentials in the device-local secrets carve-out.
• Browse: libraries → series/authors → books, plus podcasts, which Audiobookshelf handles as a first-class library type — that pairs directly with the bundled podcasts addon and gives podcast subscriptions a real home (Follow a series: scheduled update checks, new-item notifications, and optional auto-download #155's following model applies to new episodes).
• Play: stream the item, honouring its chapter structure — Audiobookshelf exposes chapters explicitly, so The listening kit: sleep timer, per-book speed memory, audio bookmarks, and jump intervals #140's chapter navigation, per-book speed memory, and sleep timer all work against a server item exactly as they do a local M4B.
• Progress is the server's: Audiobookshelf tracks listening position per user and every client syncs to it. Report position on the same throttled hook PlaybackSession already uses, and defer to the server for its own items — the identical rule Jellyfin server integration: browse, play, and report progress to an existing server #83 sets for Jellyfin and OPDS-PSE: stream comic pages from a server instead of downloading whole volumes #153 for PSE. Don't duplicate the position into our sync categories.
• Offline: download through DownloadManager into ordinary Downloaded items (Offline downloads from a Jellyfin server, transcoded variants included #110's shape), with progress reported back on reconnect via the store-and-forward queue.
• Background listening rides Background audio on mobile: keep playing with the screen off, with MediaSession controls #138 unchanged, which is the whole point — an audiobook client that stops when the screen locks isn't one.

Composes with: #139 (the local shelf, which this parallels rather than replaces), #140 (the listening kit — all of it applies), #194's merged-library layer if that lands (an audiobook owned locally and on the server is exactly the duplicate case it solves), and #195 (Audiobooks is one of the Auto shelves).

Not in scope: uploading to Audiobookshelf, its admin surface (users, library scans), and ebook items in its libraries (it stores some — but our reading stack reaches books through #134/#146, and adding a third path for them isn't worth it).