fix(web): key ChatMarkdown renderers on thread ids, not object identity - #9310

💭 This is a React component change adding useMemo with a dependency array, not Effect service code, so there's nothing to flag under Effect conventions. The import for useMemo could theoretically be missing, but that's outside the scope of this check—so it looks clear. I need to report "All clear" as the final result, though I'll need to fit that into the required title/summary fields since a bare one-line response isn't the actual output format expected.