fix: follow DSH shell dark theme for code blocks and Mermaid

[thaning0]

Problem

MarkstreamMarkdown never passed isDark to MarkdownRender, so markstream fell back to its light theme regardless of the page appearance. In a dark-themed DSH web profile:

• Shiki code blocks rendered with a white background and light-theme token colors on a dark page
• Mermaid diagrams used light-theme SVG colors inside a white panel while the rest of the chat was dark

Fix

Adds useDshIsDark() in src/client/renderer.tsx:

1. Primary signal: luminance of the computed --dsw-alias-bg-base token (threshold 140/255). This is always fresh and independent of how DSH's theme system applies tokens.
2. Fallbacks: the data-ds-dark-theme attribute on <body>, then OS preference via prefers-color-scheme.
3. Live reactivity: a debounced (80 ms) MutationObserver on <html>/<body> (data-ds-dark-theme and style attributes) re-renders when the user switches themes without reloading.

The resolved value is passed as isDark to MarkdownRender, which propagates it to Shiki code blocks, the Mermaid renderer (dark SVG theme), math containers, and the root .markstream-react.dark CSS class.

Verification

• New vitest case: renders with data-ds-dark-theme set → asserts .markstream-react.dark; removes the attribute → asserts the dark class goes away via the observer path. All 6 tests pass (pnpm run check).
• Manually verified in a DSH 0.1.0-rc.6 web profile: patched bundle served from /plugins/dsh-better-markdown/client.js; code blocks and Mermaid follow the dark page, and theme switches apply live without reload.

Notes

• No new dependencies; no API changes.
• If upstream prefers a dedicated src/client/dsh-theme.ts module or an explicit isDark prop on MarkstreamMarkdown, happy to restructure.

[coderabbitai]

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[zerob13]

LGTM