feat: Migrate binary settings to json (#920)

## Summary

* This PR introduces a migration from binary file storage to JSON-based
storage for application settings, state, and various credential stores.
This improves readability, maintainability, and allows for easier manual
configuration editing.
* Benefits:
  - Settings files are now JSON and can be easily read/edited manually
  - Easier to inspect application state and settings during development
  - JSON structure is more flexible for future changes
* Drawback: around 15k of additional flash usage
* Compatibility: Seamless migration preserves existing user data

## Additional Context
1. New JSON I/O Infrastructure files:
- JsonSettingsIO: Core JSON serialization/deserialization logic using
ArduinoJson library
- ObfuscationUtils: XOR-based password obfuscation for sensitive data
2. Migrated Components (now use JSON storage with automatic binary
migration):
     - CrossPointSettings (settings.json): Main application settings
- CrossPointState (state.json): Application state (open book, sleep
mode, etc.)
- WifiCredentialStore (wifi.json): WiFi network credentials (Password
Obfuscation: Sensitive data like WiFi passwords, uses XOR encryption
with fixed keys. Note: This is obfuscation, not cryptographic security -
passwords can be recovered with the key)
- KOReaderCredentialStore (koreader.json): KOReader sync credentials
     - RecentBooksStore (recent.json): Recently opened books list
3. Migration Logic
     - Forward Compatibility: New installations use JSON format
- Backward Compatibility: Existing binary files are automatically
migrated to JSON on first load
- Backup Safety: Original binary files are renamed with .bak extension
after successful migration
- Fallback Handling: If JSON parsing fails, system falls back to binary
loading
4. Infrastructure Updates
     - HalStorage: Added rename() method for backup operations

---

### 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: Dave Allie <dave@daveallie.com>

src/WifiCredentialStore.cpp

@@ -1,70 +1,77 @@
 #include "WifiCredentialStore.h"
 
 #include <HalStorage.h>
+#include <JsonSettingsIO.h>
 #include <Logging.h>
+#include <ObfuscationUtils.h>
 #include <Serialization.h>
 
 // Initialize the static instance
 WifiCredentialStore WifiCredentialStore::instance;
 
 namespace {
-// File format version
-constexpr uint8_t WIFI_FILE_VERSION = 2;  // Increased version
+// File format version (for binary migration)
+constexpr uint8_t WIFI_FILE_VERSION = 2;
 
-// WiFi credentials file path
-constexpr char WIFI_FILE[] = "/.crosspoint/wifi.bin";
+// File paths
+constexpr char WIFI_FILE_BIN[] = "/.crosspoint/wifi.bin";
+constexpr char WIFI_FILE_JSON[] = "/.crosspoint/wifi.json";
+constexpr char WIFI_FILE_BAK[] = "/.crosspoint/wifi.bin.bak";
 
-// Obfuscation key - "CrossPoint" in ASCII
-// This is NOT cryptographic security, just prevents casual file reading
-constexpr uint8_t OBFUSCATION_KEY[] = {0x43, 0x72, 0x6F, 0x73, 0x73, 0x50, 0x6F, 0x69, 0x6E, 0x74};
-constexpr size_t KEY_LENGTH = sizeof(OBFUSCATION_KEY);
-}  // namespace
+// Legacy obfuscation key - "CrossPoint" in ASCII (only used for binary migration)
+constexpr uint8_t LEGACY_OBFUSCATION_KEY[] = {0x43, 0x72, 0x6F, 0x73, 0x73, 0x50, 0x6F, 0x69, 0x6E, 0x74};
+constexpr size_t LEGACY_KEY_LENGTH = sizeof(LEGACY_OBFUSCATION_KEY);
 
-void WifiCredentialStore::obfuscate(std::string& data) const {
-  LOG_DBG("WCS", "Obfuscating/deobfuscating %zu bytes", data.size());
+void legacyDeobfuscate(std::string& data) {
   for (size_t i = 0; i < data.size(); i++) {
-    data[i] ^= OBFUSCATION_KEY[i % KEY_LENGTH];
+    data[i] ^= LEGACY_OBFUSCATION_KEY[i % LEGACY_KEY_LENGTH];
   }
 }
+}  // namespace
 
 bool WifiCredentialStore::saveToFile() const {
-  // Make sure the directory exists
   Storage.mkdir("/.crosspoint");
-
-  FsFile file;
-  if (!Storage.openFileForWrite("WCS", WIFI_FILE, file)) {
-    return false;
-  }
-
-  // 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
-  for (const auto& cred : credentials) {
-    // Write SSID (plaintext - not sensitive)
-    serialization::writeString(file, cred.ssid);
-    LOG_DBG("WCS", "Saving SSID: %s, password length: %zu", cred.ssid.c_str(), cred.password.size());
-
-    // Write password (obfuscated)
-    std::string obfuscatedPwd = cred.password;
-    obfuscate(obfuscatedPwd);
-    serialization::writeString(file, obfuscatedPwd);
-  }
-
-  file.close();
-  LOG_DBG("WCS", "Saved %zu WiFi credentials to file", credentials.size());
-  return true;
+  return JsonSettingsIO::saveWifi(*this, WIFI_FILE_JSON);
 }
 
 bool WifiCredentialStore::loadFromFile() {
+  // Try JSON first
+  if (Storage.exists(WIFI_FILE_JSON)) {
+    String json = Storage.readFile(WIFI_FILE_JSON);
+    if (!json.isEmpty()) {
+      bool resave = false;
+      bool result = JsonSettingsIO::loadWifi(*this, json.c_str(), &resave);
+      if (result && resave) {
+        LOG_DBG("WCS", "Resaving JSON with obfuscated passwords");
+        saveToFile();
+      }
+      return result;
+    }
+  }
+
+  // Fall back to binary migration
+  if (Storage.exists(WIFI_FILE_BIN)) {
+    if (loadFromBinaryFile()) {
+      if (saveToFile()) {
+        Storage.rename(WIFI_FILE_BIN, WIFI_FILE_BAK);
+        LOG_DBG("WCS", "Migrated wifi.bin to wifi.json");
+        return true;
+      } else {
+        LOG_ERR("WCS", "Failed to save wifi during migration");
+        return false;
+      }
+    }
+  }
+
+  return false;
+}
+
+bool WifiCredentialStore::loadFromBinaryFile() {
   FsFile file;
-  if (!Storage.openFileForRead("WCS", WIFI_FILE, file)) {
+  if (!Storage.openFileForRead("WCS", WIFI_FILE_BIN, file)) {
     return false;
   }
 
-  // Read and verify version
   uint8_t version;
   serialization::readPod(file, version);
   if (version > WIFI_FILE_VERSION) {
@@ -79,29 +86,20 @@ bool WifiCredentialStore::loadFromFile() {
     lastConnectedSsid.clear();
   }
 
-  // Read credential count
   uint8_t count;
   serialization::readPod(file, count);
 
-  // Read credentials
   credentials.clear();
   for (uint8_t i = 0; i < count && i < MAX_NETWORKS; i++) {
     WifiCredential cred;
-
-    // Read SSID
     serialization::readString(file, cred.ssid);
-
-    // Read and deobfuscate password
     serialization::readString(file, cred.password);
-    LOG_DBG("WCS", "Loaded SSID: %s, obfuscated password length: %zu", cred.ssid.c_str(), cred.password.size());
-    obfuscate(cred.password);  // XOR is symmetric, so same function deobfuscates
-    LOG_DBG("WCS", "After deobfuscation, password length: %zu", cred.password.size());
-
+    legacyDeobfuscate(cred.password);
     credentials.push_back(cred);
   }
 
   file.close();
-  LOG_DBG("WCS", "Loaded %zu WiFi credentials from file", credentials.size());
+  // LOG_DBG("WCS", "Loaded %zu WiFi credentials from binary file", credentials.size());
   return true;
 }