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.