fix: assistant text rendering, toolResult plumbing leak; add render smoke tests

- Markdown.tsx never passed text as children to ReactMarkdown (valid JSX,
  compiled clean, rendered nothing) — the 'thinking shows, text doesn't' bug
- toolResult entries (quiz/ask dispatch instructions for the LLM) leaked
  into the transcript as visible text; now never rendered
- render smoke tests (renderToStaticMarkup, no DOM): pin markdown content,
  KaTeX, button-based quiz options, ask answers not graded, toolResult and
  [Quiz answer] hiding — negative-tested against the actual bugs
- React default import in all .tsx: tsx test runner uses classic JSX
  transform (ignores tsconfig jsx option), Vite uses automatic runtime;
  explicit import works under both
- katex css moved to main.tsx (tests can't import css)
- verified live in browser: all assistant messages render, ask card has no
  grading marks, pending quiz re-arms on resume

tsconfig.json

@@ -9,6 +9,7 @@
 		"noUncheckedIndexedAccess": true,
 		"verbatimModuleSyntax": true,
 		"allowImportingTsExtensions": true,
+		"jsx": "react-jsx",
 		"noEmit": true,
 		"skipLibCheck": true
 	},