feat: add RealtimeMessage model and bump version to 3.7.7

[PHKenny]

📋 Summary

• Introduces a new realtime module with the RealtimeMessage model
• Bumps package version to 3.7.7

📝 Changes

✨ Features

• Added RealtimeMessage freezed model with topic (String) and payload (Map<String, dynamic>) fields in the new lib/src/realtime/ module
• Exported realtime module from lib/layrz_models.dart

🔧 Chore / Config

• Bumped version to 3.7.7 in pubspec.yaml
• Updated CHANGELOG.md with 3.7.7 entry

🤖 Generated with Claude Code

[github-actions]

📋 Changelog Summary

This PR includes the following changes:

✨ Features

• add RealtimeMessage model and realtime module (37e2195 by @PHKenny, Claude Sonnet 4.6)

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[github-actions]

📊 Test Coverage Report

🎯 Dart/Flutter

Metric	Value
Coverage	11.1%
Lines covered	13990 / 438
Lines missing	-13552

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