docs(frontend): confirm functional React components (Closes #15)

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frontend/docs/react-functional-components.md

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+# React components in this app
+
+All UI under `frontend/src/**/*.tsx` uses functional components with hooks.
+
+There are no `React.Component` or `class ... extends Component` patterns in the frontend tree. Non-React TypeScript classes (for example keyword classifiers under `frontend/src/ai/`) are plain domain objects and are intentionally excluded from this rule.
+
+When adding UI:
+
+- Prefer function components with `useState`, `useEffect`, and `useCallback`.
+- Keep side effects out of render paths.
+- Run `python -m unittest tools.tests.test_functional_react_components -v` before opening a PR.

tools/tests/test_functional_react_components.py

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+"""Ensure React UI files remain functional components."""
+
+from __future__ import annotations
+
+import re
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+FRONTEND_SRC = ROOT / "frontend" / "src"
+
+CLASS_COMPONENT_PATTERNS = (
+    re.compile(r"extends\s+React\.Component"),
+    re.compile(r"extends\s+Component\b"),
+    re.compile(r"class\s+\w+\s+extends\s+PureComponent"),
+)
+
+
+class FunctionalReactComponentTests(unittest.TestCase):
+    def test_no_class_components_in_frontend_src(self) -> None:
+        offenders: list[str] = []
+        for path in FRONTEND_SRC.rglob("*"):
+            if path.suffix not in {".tsx", ".jsx"}:
+                continue
+            source = path.read_text(encoding="utf-8")
+            for pattern in CLASS_COMPONENT_PATTERNS:
+                if pattern.search(source):
+                    offenders.append(str(path.relative_to(ROOT)))
+        self.assertEqual(offenders, [])
+
+
+if __name__ == "__main__":
+    unittest.main()