fix(web): show longer branch names in chat header

[wbxl2000]

Related Issue

No linked issue. This PR addresses a web UI limitation found during review.

Problem

Long branch names in the web chat header were truncated by a fixed 180px limit, and there was no hover text to reveal the full branch name.

What changed

• Let the chat header branch area use the available right-side space instead of a fixed 180px cap.
• Keep ellipsis as a fallback when the branch name is still longer than the available space.
• Add a title tooltip on the branch name so the full name remains readable on hover.
• Add tests for long branch rendering and the detached label.
• Add a changeset for the bundled web UI change.

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: 778d8ef

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]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@778d8ef

npx https://pkg.pr.new/@moonshot-ai/kimi-code@778d8ef

commit: 778d8ef