feat: Connect to last wifi by default (#752)

## Summary

* **What is the goal of this PR?** 

Use last connected network as default

* **What changes are included?**

- Refactor how an action type of Settings are handled
- Add a new System Settings option → Network
- Add the ability to forget a network in the Network Selection Screen
- Add the ability to Refresh network list
- Save the last connected network SSID
- Use the last connection whenever network is needed (OPDS, Koreader
sync, update etc)

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).


![IMG_6504](https://github.com/user-attachments/assets/e48fb013-b5c3-45c0-b284-e183e6fd5a68)

![IMG_6503](https://github.com/user-attachments/assets/78c4b6b6-4e7b-4656-b356-19d65ff6aa12)




https://github.com/user-attachments/assets/95bf34a8-44ce-4279-8cd8-f78524ce745b





---

### AI Usage

Did you use AI tools to help write this code? _** PARTIALLY: I wrote
most of it but I also used Gemini as assist.

---------

Co-authored-by: Eliz Kilic <elizk@google.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/WifiCredentialStore.h

@@ -16,6 +16,7 @@ class WifiCredentialStore {
  private:
   static WifiCredentialStore instance;
   std::vector<WifiCredential> credentials;
+  std::string lastConnectedSsid;
 
   static constexpr size_t MAX_NETWORKS = 8;
 
@@ -48,6 +49,11 @@ class WifiCredentialStore {
   // Check if a network is saved
   bool hasSavedCredential(const std::string& ssid) const;
 
+  // Last connected network
+  void setLastConnectedSsid(const std::string& ssid);
+  const std::string& getLastConnectedSsid() const;
+  void clearLastConnectedSsid();
+
   // Clear all credentials
   void clearAll();
 };