* perf: cache KaTeX module import as singleton across all renderer instances
KatexRenderer.svelte dynamically imports katex, mhchem, and the CSS on every component mount. When a message contains multiple math expressions, this triggers redundant module resolution for each one. Move the import promise to a module-level singleton using Svelte's context='module' script block so it loads once and is shared across all KatexRenderer instances.
* Update KatexRenderer.svelte