fix: validate the deep-link state param before propagating it

?state= was used verbatim and ends up interpolated into a fetch URL and into
the Hugging Face parquet shard path duckdb-wasm reads. Impact is low -- it is a
client-only fetch to a public HTTPS URL, and the wasm sandbox loads no httpfs
-- but validating at the boundary is cheap and correct. Deep links now require
/^[A-Z]{2}$/ (after trim + uppercase, so ?state=az still works) and fall back
to the state selector with a warning when they do not match.

src/App.jsx

@@ -6,9 +6,14 @@ import ChartPanel from './components/ChartPanel.jsx'
 import Footer from './components/Footer.jsx'
 import { fetchDistrictEstimates } from './hooks/useApi.js'
 
+// Two-letter USPS state code. The deep-link value flows into API request URLs
+// and into the Hugging Face parquet shard path duckdb-wasm reads, so it gets
+// validated here at the boundary rather than propagated verbatim.
+const STATE_CODE_PATTERN = /^[A-Z]{2}$/
+
 /**
  * CRDC Arrests API Demo App — main router.
- * Flow: state → district search (with interesting suggestions) → loading animation → 6 charts
+ * Flow: state → district search (with interesting suggestions) → loading animation → charts
  */
 export default function App() {
   const [step, setStep] = useState('state')   // 'state' | 'search' | 'loading' | 'results'
@@ -19,7 +24,16 @@ export default function App() {
   useEffect(() => {
     const params = new URLSearchParams(window.location.search)
     const leaid = params.get('leaid')
-    const stateParam = params.get('state')
+    const rawState = params.get('state')
+    const stateParam = rawState ? rawState.trim().toUpperCase() : null
+
+    if (rawState && !STATE_CODE_PATTERN.test(stateParam)) {
+      // Malformed deep link — drop it and start at the state selector rather
+      // than passing an arbitrary string into fetch URLs and shard paths.
+      console.warn('Ignoring deep link: `state` is not a two-letter state code.')
+      return
+    }
+
     if (leaid && stateParam) {
       // Deep link (including browser back/forward landing on this URL): resolve
       // the district name via a real estimates row, keyed by LEAID. The previous