feat: edit wifi networks in webui (#1743)

## Summary

**What is the goal of this PR?**
Add Wi-Fi network management to the Web UI settings page, similar to
existing OPDS server management, so users can view, add, edit, and
delete saved Wi-Fi credentials from the browser. Closes
https://github.com/crosspoint-reader/crosspoint-reader/issues/1544 and
https://github.com/crosspoint-reader/crosspoint-reader/discussions/607

**What changes are included?**

- Added Wi-Fi API endpoints:
- GET /api/wifi Listing saved networks (without exposing plaintext
passwords)
  - POST /api/wifi Creating/updating networks
  - POST /api/wifi/delete Deleting networks by index

- Added a new Wi-Fi Networks management section web UI settings page

<img width="921" height="712" alt="image"
src="https://github.com/user-attachments/assets/42c20a63-f335-4389-8246-d38065d6b65a"
/>


---

### 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 >**_

---------

Co-authored-by: Copilot <copilot@github.com>

USER_GUIDE.md

@@ -21,7 +21,8 @@ Welcome to the **CrossPoint** firmware. This guide outlines the hardware control
       - [3.6.3 Controls](#363-controls)
       - [3.6.4 System](#364-system)
       - [3.6.5 OPDS Servers (Multiple Libraries)](#365-opds-servers-multiple-libraries)
-      - [3.6.6 KOReader Sync Quick Setup](#366-koreader-sync-quick-setup)
+      - [3.6.6 Web Settings (WiFi + OPDS)](#366-web-settings-wifi--opds)
+      - [3.6.7 KOReader Sync Quick Setup](#367-koreader-sync-quick-setup)
     - [3.7 Sleep Screen](#37-sleep-screen)
   - [4. Reading Mode](#4-reading-mode)
     - [Page Turning](#page-turning)
@@ -223,7 +224,24 @@ You can also manage OPDS servers from the web interface while in File Transfer m
 2. Open `http://<device-ip>/settings`.
 3. Use the **OPDS Servers** card to add, edit, or delete entries.
 
-#### 3.6.6 KOReader Sync Quick Setup
+For web-based WiFi network management, see [Web Settings (WiFi + OPDS)](#366-web-settings-wifi--opds).
+
+#### 3.6.6 Web Settings (WiFi + OPDS)
+
+While in **File Transfer** mode, the web settings page includes management cards for both **WiFi Networks** and **OPDS Servers**.
+
+1. On device: open **File Transfer** and connect to WiFi.
+1. In a browser, open `http://<device-ip>/settings` or `http://crosspoint.local`.
+1. In **WiFi Networks**, add, edit, or delete saved network entries (SSID + optional password).
+1. In **OPDS Servers**, add, edit, or delete OPDS catalogs.
+
+Behavior notes:
+
+- Passwords are never shown back in the web UI after saving.
+- Leaving Password blank while editing keeps the existing saved password unchanged.
+- The web UI can save hidden-network SSIDs, but connecting to hidden networks still depends on device-side WiFi connection flow.
+
+#### 3.6.7 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.