chore: Refactor stores to use PersistableStore CRTP template (#2464)

src/WifiCredentialStore.h

@@ -1,4 +1,7 @@
 #pragma once
+#include <ArduinoJson.h>
+#include <PersistableStore.h>
+
 #include <string>
 #include <vector>
 
@@ -7,21 +10,14 @@ struct WifiCredential {
   std::string password;  // Plaintext in memory; obfuscated with hardware key on disk
 };
 
-class WifiCredentialStore;
-namespace JsonSettingsIO {
-bool saveWifi(const WifiCredentialStore& store, const char* path);
-bool loadWifi(WifiCredentialStore& store, const char* json, bool* needsResave);
-}  // namespace JsonSettingsIO
-
 /**
  * Singleton class for storing WiFi credentials on the SD card.
  * Passwords are XOR-obfuscated with the device's unique hardware MAC address
  * and base64-encoded before writing to JSON (not cryptographically secure,
  * but prevents casual reading and ties credentials to the specific device).
  */
-class WifiCredentialStore {
+class WifiCredentialStore : public PersistableStore<WifiCredentialStore> {
  private:
-  static WifiCredentialStore instance;
   std::vector<WifiCredential> credentials;
   std::string lastConnectedSsid;
 
@@ -30,22 +26,12 @@ class WifiCredentialStore {
   // Private constructor for singleton
   WifiCredentialStore() = default;
 
-  bool loadFromBinaryFile();
-
-  friend bool JsonSettingsIO::saveWifi(const WifiCredentialStore&, const char*);
-  friend bool JsonSettingsIO::loadWifi(WifiCredentialStore&, const char*, bool*);
+  friend class PersistableStore<WifiCredentialStore>;
 
  public:
-  // Delete copy constructor and assignment
-  WifiCredentialStore(const WifiCredentialStore&) = delete;
-  WifiCredentialStore& operator=(const WifiCredentialStore&) = delete;
-
-  // Get singleton instance
-  static WifiCredentialStore& getInstance() { return instance; }
-
-  // Save/load from SD card
-  bool saveToFile() const;
-  bool loadFromFile();
+  static const char* getFilePath() { return "/.crosspoint/wifi.json"; }
+  void toJson(JsonDocument& doc) const;
+  bool fromJson(JsonVariantConst doc);
 
   // Credential management
   bool addCredential(const std::string& ssid, const std::string& password);