feat: mailio-based POP3 synchronizer (option USE_MAILIO_IMAP)

- Added Pop3SynchronizerMailio using mailio library
- CMake option USE_MAILIO_IMAP controls both IMAP and POP3 mailio synchronizers
- Implements POP3: connect, auth (USER/PASS, XOAUTH2), LIST, RETR, DELE
- SyncFolder: fetches new messages by UID
- XOAUTH2 support for Gmail/Outlook OAuth2 tokens
- Falls back to original Pop3Synchronizer when USE_MAILIO_IMAP=OFF
- Ready to enable with: cmake -DUSE_MAILIO_IMAP=ON

CMakeLists.txt

@@ -127,6 +127,7 @@ set(SRC_FILES
 if (USE_MAILIO_IMAP)
     list(APPEND SRC_FILES
         src/services/imap/imapsynchronizer_mailio.cpp
+        src/services/pop3/pop3synchronizer_mailio.cpp
     )
 endif()
 # Executable