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.cpp

@@ -9,7 +9,7 @@ WifiCredentialStore WifiCredentialStore::instance;
 
 namespace {
 // File format version
-constexpr uint8_t WIFI_FILE_VERSION = 1;
+constexpr uint8_t WIFI_FILE_VERSION = 2;  // Increased version
 
 // WiFi credentials file path
 constexpr char WIFI_FILE[] = "/.crosspoint/wifi.bin";
@@ -38,6 +38,7 @@ bool WifiCredentialStore::saveToFile() const {
 
   // Write header
   serialization::writePod(file, WIFI_FILE_VERSION);
+  serialization::writeString(file, lastConnectedSsid);  // Save last connected SSID
   serialization::writePod(file, static_cast<uint8_t>(credentials.size()));
 
   // Write each credential
@@ -67,12 +68,18 @@ bool WifiCredentialStore::loadFromFile() {
   // Read and verify version
   uint8_t version;
   serialization::readPod(file, version);
-  if (version != WIFI_FILE_VERSION) {
+  if (version > WIFI_FILE_VERSION) {
     Serial.printf("[%lu] [WCS] Unknown file version: %u\n", millis(), version);
     file.close();
     return false;
   }
 
+  if (version >= 2) {
+    serialization::readString(file, lastConnectedSsid);
+  } else {
+    lastConnectedSsid.clear();
+  }
+
   // Read credential count
   uint8_t count;
   serialization::readPod(file, count);
@@ -128,6 +135,9 @@ bool WifiCredentialStore::removeCredential(const std::string& ssid) {
   if (cred != credentials.end()) {
     credentials.erase(cred);
     Serial.printf("[%lu] [WCS] Removed credentials for: %s\n", millis(), ssid.c_str());
+    if (ssid == lastConnectedSsid) {
+      clearLastConnectedSsid();
+    }
     return saveToFile();
   }
   return false;  // Not found
@@ -146,8 +156,25 @@ const WifiCredential* WifiCredentialStore::findCredential(const std::string& ssi
 
 bool WifiCredentialStore::hasSavedCredential(const std::string& ssid) const { return findCredential(ssid) != nullptr; }
 
+void WifiCredentialStore::setLastConnectedSsid(const std::string& ssid) {
+  if (lastConnectedSsid != ssid) {
+    lastConnectedSsid = ssid;
+    saveToFile();
+  }
+}
+
+const std::string& WifiCredentialStore::getLastConnectedSsid() const { return lastConnectedSsid; }
+
+void WifiCredentialStore::clearLastConnectedSsid() {
+  if (!lastConnectedSsid.empty()) {
+    lastConnectedSsid.clear();
+    saveToFile();
+  }
+}
+
 void WifiCredentialStore::clearAll() {
   credentials.clear();
+  lastConnectedSsid.clear();
   saveToFile();
   Serial.printf("[%lu] [WCS] Cleared all WiFi credentials\n", millis());
 }