Smart Review: instant loading feedback, no more dead-feeling tap

Reported: tapping Review due could sit for many seconds with no loading
screen. The flow itself is local and fast (counts are hydrated on the
library; <1.5s even at 6x CPU throttle with 3,000 answers), so the
visible fault was missing feedback: /review is a dynamic route and
until its payload arrived nothing on screen changed.

- LinkLoadingScreen: the review card links render a full-screen
  loading screen from the link's own useLinkStatus pending state, the
  instant the tap happens (measured 58 ms with the route request held
  open). The links also prefetch their payload.
- Route loading boundaries (loading.tsx) for /review, /stats and
  /exam, sharing a RouteLoading screen (app icon + spinner +
  localized message); ReviewClient's bare skeleton replaced with it.
- Health probes: per-key 30s cache + in-flight sharing in use-health.ts
  (every page mounts its own gate; a session now fires one probe
  instead of one per navigation). Manual re-check still forces.
- Czech fix: exam heading 'Zkouškové režim' -> 'Zkouškový režim'.

Verified by the rebuilt suites (the /tmp harness was wiped overnight):
review-loading 15/15 (stalled-request loading screen at 58 ms, session
arrival after release, fast normal path, mistakes path, end-to-end
grading, probe cache) and navigation 78/78 (tab bar, labels 360/390
EN/CS, routing, deep links, wizard loading lock). Lint + build clean.
SW freezmo-v30.

lib/use-health.ts

@@ -9,23 +9,55 @@ import { settingsStore } from "./settings";
 /**
  * Health status for the current browser (re-checks whenever the API key
  * changes). Used by the settings dialog and the API-key gate.
+ *
+ * Every page mounts its own gate, and one probe can sit for seconds while
+ * Ollama answers — so probes are deduplicated per key and cached briefly
+ * instead of firing one per mount. The manual re-check always forces.
  */
+const HEALTH_TTL_MS = 30_000;
+
+let cache: { key: string; at: number; value: Health } | null = null;
+let inflight: { key: string; promise: Promise<Health> } | null = null;
+
+function loadHealth(key: string, force: boolean): Promise<Health> {
+  if (!force && cache && cache.key === key && Date.now() - cache.at < HEALTH_TTL_MS) {
+    return Promise.resolve(cache.value);
+  }
+  if (!force && inflight && inflight.key === key) return inflight.promise;
+
+  const promise = fetchHealth()
+    .then((value) => {
+      cache = { key, at: Date.now(), value };
+      return value;
+    })
+    .finally(() => {
+      if (inflight?.promise === promise) inflight = null;
+    });
+  inflight = { key, promise };
+  return promise;
+}
+
 export function useHealth() {
   const apiKey = useSyncExternalStore(
     settingsStore.subscribe,
     () => settingsStore.getSnapshot().apiKey,
     () => "",
   );
-  const [health, setHealth] = useState<Health | null>(null);
+  const [health, setHealth] = useState<Health | null>(() =>
+    cache && cache.key === apiKey ? cache.value : null,
+  );
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
-  const [checkedAt, setCheckedAt] = useState<number | null>(null);
+  const [checkedAt, setCheckedAt] = useState<number | null>(() =>
+    cache && cache.key === apiKey ? cache.at : null,
+  );
 
-  const refresh = useCallback(async () => {
+  const run = useCallback(async (force: boolean) => {
     setLoading(true);
     setError(null);
     try {
-      setHealth(await fetchHealth());
+      const key = settingsStore.getSnapshot().apiKey;
+      setHealth(await loadHealth(key, force));
     } catch (loadError) {
       setError(loadError instanceof Error ? loadError.message : t("api.unreachableShort"));
     } finally {
@@ -36,8 +68,11 @@ export function useHealth() {
   }, []);
 
   useEffect(() => {
-    void refresh();
-  }, [refresh, apiKey]);
+    void run(false);
+  }, [run, apiKey]);
+
+  /** The settings dialog's re-check button: always a fresh probe. */
+  const refresh = useCallback(() => run(true), [run]);
 
   return { health, loading, error, refresh, apiKey, checkedAt };
 }