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>

src/network/CrossPointWebServer.h

@@ -112,4 +112,9 @@ class CrossPointWebServer {
   void handleGetOpdsServers() const;
   void handlePostOpdsServer();
   void handleDeleteOpdsServer();
+
+  // Wi-Fi credential handlers
+  void handleGetWifiNetworks() const;
+  void handlePostWifiNetwork();
+  void handleDeleteWifiNetwork();
 };