Refine generation pipeline and remove layout debug clutter

[leoafarias]

Summary

• Tighten the AI deck generation pipeline with updated Gemini model usage, prompt shaping, repair/validation flow, and quality reporting.
• Clean up the playground generation lab UI and related docs/configuration.
• Improve slide rendering and capture behavior by simplifying layout debug handling and updating styling defaults.
• Refresh tests and goldens to cover the new generation, validation, and rendering behavior.

Testing

• Updated and added unit tests for prompt generation, deck customization, validators, quality reporting, and source grounding.
• Ran live generation smoke coverage for the playground pipeline.
• Refreshed rendering goldens for slide and block layout cases.

[github-actions]

Visit the preview URL for this PR (updated for commit 559c78c):

https://superdeck-dev--pr102-chore-cleanup-debug-iphr0z6h.web.app

_{(expires Sat, 15 Aug 2026 13:37:07 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: bd68fc230762285849207e7e120aaf87cd4ca2f9}