cross-cutting: unified onUnreadCountChange with polling fallback

[productdevbook]

Unread count APIs are inconsistent: Intercom `onUnreadCountChange`, Chatwoot CustomEvent, HelpScout `Beacon('on', 'chat-message-count-change')`, Drift `drift.on('message')` only, Olark/Tidio event-based, Smartsupp no API.

Evidence

• `Get Unread Count via Hook devrnt/react-use-intercom#479` — "Get unread count via hook" Get Unread Count via Hook devrnt/react-use-intercom#479

Tasks

• `onUnreadCountChange(count: number) => void` at top-level API
• Per adapter: native event if available
• Providers without native support: polling loop (backoff 5s→30s, pause when document hidden)

[productdevbook]

Implemented in this commit