Implement IMAP IDLE command for real-time mailbox notifications, allowing clients to receive immediate push updates about new, deleted, and changed messages without polling.
Changes:
include/mailio/imap.hpp:

- Add idle_notification_type_t enum (EXISTS, EXPUNGE, FETCH)
- Add idle_response_t struct for parsed server notifications
- Add capability() / has_capability() methods for server capability discovery
- Add idle() / idle_done() method declarations with private state members

src/imap.cpp:

- Implement capability() — fetch and cache CAPABILITY response
- Implement has_capability() — check for specific capability string
- Implement idle() — enter IDLE mode, parse untagged notifications (EXISTS, EXPUNGE, FETCH), handle configurable timeout (default 29 min per RFC, recommended ~10 min for Gmail)
- Implement idle_done() — send DONE to terminate IDLE; safe to call from another thread

examples/imap_idle.cpp:

- Add usage example demonstrating IDLE connection, notification handling, and graceful shutdown

Protocol details:

- Command sequence: tag IDLE → + continuation → untagged notifications → DONE → tag OK
- Requires server IDLE capability advertisement
- Must be in Selected mailbox state
- Thread-safe design: idle_done() can be called from a separate thread to interrupt idle()