updated audiobook player guide

Audiobook Player Guide.md

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+## Audiobook Player (Audio variant only)
+
+Press **P** from the home screen to open the audiobook player.
+Place `.m4b`, `.m4a`, `.mp3`, or `.wav` files in `/audiobooks/` on the SD card.
+
+| Key | Action |
+|-----|--------|
+| W / S | Scroll file list / Volume up-down |
+| Enter | Select book / Play-Pause |
+| A | Seek back 30 seconds |
+| D | Seek forward 30 seconds |
+| [ | Previous chapter (M4B only) |
+| ] | Next chapter (M4B only) |
+| Q | Leave player (audio continues) / Close book (when paused) / Exit (from file list) |
+
+**Bookmarks** are saved automatically every 30 seconds during playback and when
+you stop or exit. Reopening a book resumes from your last position.
+
+**Cover art** from M4B files is displayed as dithered monochrome on the e-ink
+screen, along with title, author, and chapter information.
+
+### Background Playback
+
+Audio continues playing when you leave the audiobook player screen. Press **Q**
+while audio is playing to return to the home screen — a **>>** indicator will
+appear in the status bar next to the battery icon to show that audio is active
+in the background. Press **P** at any time to return to the player screen and
+resume control.
+
+If you pause or stop playback first and then press **Q**, the book is closed
+and you're returned to the file list instead.
+
+### Audio Hardware
+
+The audiobook player uses the PCM5102A I2S DAC on the audio variant of the
+T-Deck Pro (I2S pins: BCLK=7, DOUT=8, LRC=9). Audio is output via the 3.5mm
+headphone jack.
+
+> **Note:** The audiobook player is not available on the 4G modem variant
+> due to I2S pin conflicts. MP3 format is recommended over M4B for best
+> compatibility with the ESP32-audioI2S library.
+
+### SD Card Folder Structure
+
+```
+SD Card
+├── audiobooks/
+│   ├── .bookmarks/          (auto-created, stores resume positions)
+│   │   ├── mybook.bmk
+│   │   └── another.bmk
+│   ├── mybook.m4b
+│   ├── another.m4b
+│   └── podcast.mp3
+├── books/                   (existing — text reader)
+│   └── ...
+└── ...
+```