feat: Searchable runtime image dropdown with defaults for per-step image selection

### Feature Area

/area frontend
/area backend

### What feature would you like to see?

Add a searchable runtime image selector for per-step container images, while keeping support for free-text input.

Kale should provide:

- A searchable dropdown with prefilled runtime images
- Ability to still manually type an image when needed
- Support for a default runtime image by notebook, automatically selected
- Ability to define the default image inside the container (so environments can ship sensible defaults)

This builds on the existing per-step runtime image support and improves usability and standardization.

### What is the use case or pain point?

Manually typing Docker image names per step is error-prone and slow, especially when teams already have a known set of supported images. A searchable dropdown improves speed, correctness, and discoverability, while still allowing advanced users full flexibility.

### Is there a workaround currently?

Yes. Users manually type image names in step metadata or copy/paste them from examples. There is no catalog, search, validation, or default mechanism.

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Love this idea? Give it a 👍.

Start by tracing the existing per-step runtime image support and the step metadata path described in the issue. Check how notebook-level defaults and container-provided defaults could fit alongside the searchable selector and free-text input. Done means users can search prefilled images, enter custom images, and receive the appropriate notebook or container default.

Written by the indexing model from the issue text.