fix(web): keep long mermaid labels inside their nodes

[djfch]

Related Issue

Resolve #2577

Problem

See linked issue.

What changed

Long mermaid labels overflowed their node boxes in the web UI: mermaid wraps long labels with CSS soft wrap inside <foreignObject> and sizes the node rect from the wrapped measurement, but the SVG sanitizer (stream-markdown-parser, pulled in by markstream-vue) flattens every <foreignObject> into a single-line <text>, keeping only literal <br> breaks. The full-width single-line label then renders centered on a rect sized for the wrapped width, overflowing both sides and overlapping neighbors.

This PR preprocesses chat markdown before rendering: every ```mermaid fence gets an injected %%{init: {"htmlLabels": false}}%% directive (behind a leading %% comment line so markstream still prepends its theme directive, which it only does when the code does not already start with a directive). With htmlLabels off, mermaid lays out native SVG text with real width-based line breaking (tspans), so the sanitizer has no <foreignObject> left to flatten and measurement and rendering agree.

Notes:

• The directive must use the global htmlLabels key — flowchart.htmlLabels is deprecated and silently shadowed by the global default in mermaid 11.15 (verified empirically; the scoped key changes nothing).
• The injector is a pure function with unit tests covering indented/CRLF/~~~ fences, streaming partial input, diagrams carrying their own init directive, and the invariant that injected code never starts with %%{ (which would disable markstream's theme prepend).
• Verified in a real browser with the app's exact dependency versions (mermaid 11.15.0 + stream-markdown-parser 1.1.7): previously a 35-char CJK label rendered 576px of text in a 260px node; after the fix every label wraps into tspans and stays inside its rect, with dark/light theming intact.
• No upstream code is patched, so future markstream/mermaid upgrades cannot silently conflict; once the upstream sanitizer handles soft-wrapped labels, deleting the single injection call site reverts this.

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.

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🦋 Changeset detected

Latest commit: 74e0715

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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