Feat/ai summary generation

[aprabou]

feat: add AI-generated notebook summary button and first-cell insertion

• Add Summary action to the navbar and wire the UI event flow.
• Implement the frontend handler to call the summary API, refresh the notebook, and focus the first cell.
• Add the /generate_notebook_summary backend endpoint with existing AI config and billing error handling.
• Add notebook-level generate_notebook_summary() logic to:
  • generate a markdown summary from notebook context
  • insert a new first markdown cell when missing
  • update an existing auto-generated summary when present
  • tag the summary cell with metadata.plainbook_summary = true
• Add provider support for notebook summaries in Gemini and Claude integrations.
• Add shared summary system instructions and a markdown-fence stripping helper.
• Register the new summarize_notebook provider hook in the AI provider registry.

Reference: Issue #21