Add message-count alert feature (20 messages per chat)

Feature Request

Add an alert/notification when a conversation reaches 20 messages across all Claude products.

Motivation

• Helps users stay aware of conversation length for context window management
• Useful for deciding when to start a new conversation or summarize
• Applicable to web (claude.ai), Claude Code, mobile, and other Claude interfaces

Requested behavior

When a conversation reaches 20 messages (or user-configurable threshold), show a non-intrusive alert notifying the user of the message count.

Notes

This feature has already been implemented for Claude Code via hooks in settings.json, but users want it as a built-in feature across all Claude products.