Refactoring/mifare keys handling

[Senape3000]

Proposed Changes

Refactored MIFARE keys management by separating storage from main config (bruce.conf) into dedicated file (/BruceRFID/keys.conf). Fixes SD card path bug and improves performance with human-readable format.

Types of Changes

• Bugfix: SD card directory creation before file operations
• New Feature: Dedicated keys file with comment support
• Performance optimization: Append-only writes, reduced config size by 50%

Verification

1. First boot creates /BruceRFID/keys.conf with 3 standard keys
2. Add custom key via UI and verify append (no full rewrite)
3. Test duplicate rejection and hex validation
4. Verify SD ↔ LittleFS sync and fallback
5. Test RFID authentication with loaded keys

Testing

✅ Full testing completed ([file:19]):

• First boot, key addition, duplicate/invalid rejection
• SD/LittleFS fallback scenarios
• Manual file editing support
• PN532 module integration verified
• Backward compatibility confirmed

Linked Issues

Resolves SD card path bug where keys file was created in root instead of /BruceRFID/ directory.

User-Facing Change

MIFARE keys now stored in `/BruceRFID/keys.conf` (human-readable text file). Users can manually edit keys with any text editor. Automatic migration from old config format. No action required.
Further Comments
Zero breaking changes. Modified files: src/core/config.h, src/core/config.cpp. Full backward compatibility with existing RFID modules maintained

[MIFARE_KEYS_UPDATE.md](https://github.com/user-attachments/files/24414642/MIFARE_KEYS_UPDATE.md)

[Senape3000]

Rewritten, the old version took up unnecessary space in config, now it's cleaner, more maintainable and clear. It has fallbacks for various eventualities. Already tested.