feat: add optional AI-powered summaries for saved chats

[manwithshit]

Summary

This PR adds an optional AI-powered summary feature to Chat Save, allowing users to generate insights and key quotes when saving conversations.

New Features

• 🤖 AI Summary (optional, disabled by default)

  • Generate 💡 Key Insights (2-4 bullet points)
  • Extract ✨ Notable Quotes (1-3 memorable quotes)
  • Create 📋 Summary (2-3 sentence overview)

• ⚙️ Settings Page

  • Configure API key for your preferred AI provider
  • Support for OpenAI (GPT-4o-mini), Anthropic (Claude 3 Haiku), and Google (Gemini Flash)
  • Choose summary language (auto-detect, English, Chinese, Japanese)

• 📝 Enhanced Metadata

  • Provider info added to exported files
  • Smarter filename generation already in place

Privacy Considerations

• AI summary is opt-in and disabled by default
• API keys are stored locally in Chrome sync storage
• Without AI summary enabled, behavior is unchanged (fully local)

Files Changed

File	Description
options.html/js	New settings page
lib/ai-summary.js	AI summary service supporting multiple providers
lib/markdown.js	Updated to include summary in output
content-script.js	Updated save handlers to use async summary
popup.html/js/css	Added settings link
manifest.json	Added storage permission and API host permissions
README.md	Updated documentation

Test Plan

• Install extension in Chrome (Developer mode)
• Test saving without AI summary (should work as before)
• Enable AI summary in settings with valid API key
• Test saving with AI summary enabled
• Verify summary appears in exported Markdown
• Test with different AI providers
• Test error handling when API key is invalid