feat(web): group default model dropdown by provider in settings

[wbxl2000]

Related Issue

No related issue — this is a small UX improvement.

Problem

In the web settings dialog, the default model selector was a flat alphabetical list. When many models from different providers are available, it is hard to scan and pick a model by provider.

What changed

• Grouped the default model <select> options by provider using <optgroup>.
• Providers are sorted alphabetically; models within each provider are sorted by display name.
• Added a test to verify the dropdown renders provider groups and model options correctly.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: a9fdf9b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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[pkg-pr-new]

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commit: a9fdf9b