… reference

- Updated billing disclaimer to mention gemini-2.5-flash-image free tier
- Fixed audiobook section incorrectly referencing 'The Wind in the Willows' instead of 'The Adventures of Chatterer the Red Squirrel'

…formatting

The notebook_edit tool had stripped 'outputs' and 'execution_count' fields
from 14 code cells, causing GitHub to fail to render the .ipynb file.
Also restored the original 2-space JSON indentation.

Replaced all references to 'Gemini 2.5 Image' with 'Nano Banana' since the
notebook now supports multiple image models (nano-banana, nano-banana-2,
nano-banana-pro). Also fixed the broken prompt guide URL.

…ni-generated narration

The chapters list only has 'name' and 'prompt' keys from the Pydantic model.
Instead of referencing a non-existent 'full_text' field, we now ask Gemini
to write a short dramatic narration for the chapter opening, then pass that
to TTS. Also fixed 18 code cells missing 'outputs' field for nbformat compliance.

…stration