docs: add quick KOReader sync setup guide (#1181)

## Summary

* **What is the goal of this PR?**
Improve KOReader Sync documentation so new users can self-host and
configure sync quickly, with clear references in both README and
USER_GUIDE.

* **What changes are included?**
- Add a KOReader Sync feature mention in `README.md` and link to a
quick-setup section in the guide.
- Add `3.6.5 KOReader Sync Quick Setup` to `USER_GUIDE.md` with:
- Docker Compose-first setup instructions (plus Podman compose
alternative)
  - healthcheck verification
  - one-time user registration example using `curl`
- device-side setup steps (`Settings -> System -> KOReader Sync` and
`Authenticate`)
  - in-reader usage via `Sync Progress` in the reader menu
- Update reading mode navigation wording from "Chapter Menu" to "Reader
Menu" so it reflects current UI behavior.

Closes #1032.

## Additional Context

* This is documentation-only and does not change firmware behavior.
* The quick-setup flow is intentionally short and focused on getting
sync working quickly for typical home-network setups.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**YES**_

README.md

@@ -36,6 +36,7 @@ This project is **not affiliated with Xteink**; it's built as a community projec
   - [x] Cover sleep screen
 - [x] Wifi book upload
 - [x] Wifi OTA updates
+- [x] KOReader Sync integration for cross-device reading progress
 - [x] Configurable font, layout, and display options
   - [ ] User provided fonts
   - [ ] Full UTF support
@@ -43,7 +44,8 @@ This project is **not affiliated with Xteink**; it's built as a community projec
 
 Multi-language support: Read EPUBs in various languages, including English, Spanish, French, German, Italian, Portuguese, Russian, Ukrainian, Polish, Swedish, Norwegian, [and more](./USER_GUIDE.md#supported-languages).
 
-See [the user guide](./USER_GUIDE.md) for instructions on operating CrossPoint. 
+See [the user guide](./USER_GUIDE.md) for instructions on operating CrossPoint, including the
+[KOReader Sync quick setup](./USER_GUIDE.md#365-koreader-sync-quick-setup).
 
 For more details about the scope of the project, see the [SCOPE.md](SCOPE.md) document.
 

USER_GUIDE.md

@@ -20,6 +20,7 @@ Welcome to the **CrossPoint** firmware. This guide outlines the hardware control
       - [3.6.2 Reader](#362-reader)
       - [3.6.3 Controls](#363-controls)
       - [3.6.4 System](#364-system)
+      - [3.6.5 KOReader Sync Quick Setup](#365-koreader-sync-quick-setup)
     - [3.7 Sleep Screen](#37-sleep-screen)
   - [4. Reading Mode](#4-reading-mode)
     - [Page Turning](#page-turning)
@@ -197,6 +198,113 @@ The Settings screen allows you to configure the device's behavior. There are a f
 - **Check for updates**: Check for Crosspoint firmware updates over WiFi.
 - **Language**: Set the system language (see **[Supported Languages](#supported-languages)** for more information).
 
+#### 3.6.5 KOReader Sync Quick Setup
+
+CrossPoint can sync reading progress with KOReader-compatible sync servers.
+It also interoperates with KOReader apps/devices when they use the same server and credentials.
+
+##### Option A: Free Public Server (`sync.koreader.rocks`)
+
+1. Register a user once (only if needed):
+
+```bash
+USERNAME="user"
+PASSWORD="pass"
+PASSWORD_MD5="$(printf '%s' "$PASSWORD" | openssl md5 | awk '{print $2}')"
+
+curl -i "https://sync.koreader.rocks/users/create" \
+  -H "Accept: application/vnd.koreader.v1+json" \
+  -H "Content-Type: application/json" \
+  --data "{\"username\":\"$USERNAME\",\"password\":\"$PASSWORD_MD5\"}"
+```
+
+Already have KOReader Sync credentials? Skip registration; basic sync only requires using the same existing username/password on all devices.
+
+When this returns `HTTP 402` with `{"code":2002,"message":"Username is already registered."}`, pick a different username or use that existing account.
+
+2. On each CrossPoint device:
+   - Go to **Settings -> System -> KOReader Sync**.
+   - Set **Username** and **Password** (enter the plain password; CrossPoint computes MD5 internally, and use the same values on all devices).
+   - Set **Sync Server URL** to `https://sync.koreader.rocks`, or leave it empty (both use the same default KOReader sync server).
+   - Run **Authenticate**.
+
+3. While reading, press **Confirm** to open the reader menu, then select **Sync Progress**.
+   - Choose **Apply Remote** to jump to remote progress.
+   - Choose **Upload Local** to push current progress.
+
+##### Option B: Self-Hosted Server (Docker Compose)
+
+1. Start a sync server:
+
+```bash
+mkdir -p kosync-quickstart
+cd kosync-quickstart
+
+cat > compose.yaml <<'YAML'
+services:
+  kosync:
+    image: koreader/kosync:latest
+    ports:
+      - "7200:7200"
+      - "17200:17200"
+    volumes:
+      - ./data/redis:/var/lib/redis
+    environment:
+      - ENABLE_USER_REGISTRATION=true
+    restart: unless-stopped
+YAML
+
+# Docker
+docker compose up -d
+
+# Podman (alternative)
+podman compose up -d
+```
+
+> [!NOTE]
+> `ENABLE_USER_REGISTRATION=true` is convenient for first setup. After creating your users, set it to `false` (or remove it) to avoid unexpected registrations.
+
+2. Verify the server:
+
+```bash
+curl -H "Accept: application/vnd.koreader.v1+json" "http://<server-ip>:17200/healthcheck"
+# Expected: {"state":"OK"}
+```
+
+3. Register a user once.
+CrossPoint authenticates against KOReader Sync (`koreader/kosync`) using an MD5 key, so register using the MD5 of your password:
+
+> [!WARNING]
+> Sending a reusable MD5-derived password over plain HTTP is insecure.
+> Create unique sync-only credentials and do not reuse main account passwords.
+> Prefer `https://<server-ip>:7200` whenever traffic leaves a fully trusted LAN or when using untrusted networks.
+> Use `curl -k` only for self-signed certificate testing.
+
+```bash
+USERNAME="user"
+PASSWORD="pass"
+PASSWORD_MD5="$(printf '%s' "$PASSWORD" | openssl md5 | awk '{print $2}')"
+
+curl -i "http://<server-ip>:17200/users/create" \
+  -H "Accept: application/vnd.koreader.v1+json" \
+  -H "Content-Type: application/json" \
+  --data "{\"username\":\"$USERNAME\",\"password\":\"$PASSWORD_MD5\"}"
+```
+
+If this returns `HTTP 402` with `{"code":2002,"message":"Username is already registered."}`, the account already exists.
+
+4. On each CrossPoint device:
+   - Go to **Settings -> System -> KOReader Sync**.
+   - Set **Username** and **Password** (enter the plain password; CrossPoint computes MD5 internally, and use the same values on all devices).
+   - Set **Sync Server URL** to `http://<server-ip>:17200`.
+   - Run **Authenticate**.
+
+If you use the HTTPS listener, use `https://<server-ip>:7200` (`curl -k` only for self-signed certificate testing).
+
+5. While reading, press **Confirm** to open the reader menu, then select **Sync Progress**.
+   - Choose **Apply Remote** to jump to remote progress.
+   - Choose **Upload Local** to push current progress.
+
 ### 3.7 Sleep Screen
 
 You can customize the sleep screen by placing custom images in specific locations on the SD card: