feat(whatsapp): rewrite service using tulir/whatsmeow

[Jakeyzerk]

Summary

Replaces the abandoned Rhymen/go-whatsapp with tulir/whatsmeow to support WhatsApp's current Multi-Device protocol. This fixes the broken WhatsApp service.

Closes #274

Changes

• Authentication: Supports both QR code (scannable from Android, iOS, Windows app, or WhatsApp Web) and 8-digit pairing code via LoginWithPairingCode() — useful for headless servers with no display.
• Dependencies: Uses modernc.org/sqlite for CGO-free builds. Tested on Termux/Android
• Session: dbPath is an explicit parameter in both login methods. No default file is created
• Interface: Implements notify.Notifier with Send(), AddReceivers()
• Helpers: Added Disconnect() and IsConnected() for connection management
• Docs: Updated service/whatsapp/README.md with new usage examples
• Multi-Device: Uses whatsmeow's Multi-Device protocol, phone does not need to stay online after pairing. Session persists across restarts via SQLite.

Notes for Maintainer

Per discussion in #274, dbPath is currently explicit. Awaiting decision on whether to add a default path whatsmeow.db or keep it explicit to prevent unintended file creation.

New() returns *Service without error because the constructor cannot fail. Actual initialization happens lazily in LoginWithQRCode or LoginWithPairingCode. Happy to change to (*Service, error) if consistency with other services is preferred.

Testing

Tested manual login + send message on Android/Termux. Session reconnects correctly after process restart without re-authenticating.

Checklist

• My code follows the code style of this project
• I have updated the documentation accordingly
• I have added tests to cover my changes. Manual test only, no unit tests yet
• All new and existing tests passed. Manual test passed
• I have read the Contributing Guidelines