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.

components/RouteLoading.tsx

@@ -0,0 +1,31 @@
+"use client";
+
+import { Spinner } from "@/components/ui";
+import { useI18n } from "@/lib/i18n";
+
+/**
+ * Route-level loading screen (used by `loading.tsx` boundaries and by client
+ * screens that wait for local data). Rendered the moment a navigation starts,
+ * so a slow request never looks like a dead tap.
+ */
+export function RouteLoading({ messageKey }: { messageKey: string }) {
+  const { t } = useI18n();
+
+  return (
+    <div role="status" aria-busy="true" className="frost-bg grid min-h-dvh place-items-center px-4">
+      <div className="flex flex-col items-center gap-4 text-center">
+        {/* eslint-disable-next-line @next/next/no-img-element -- static app icon, no optimisation needed */}
+        <img
+          src="/icons/icon-192.png"
+          alt=""
+          aria-hidden
+          className="size-16 rounded-[1.15rem] shadow-lift motion-safe:animate-pulse"
+        />
+        <p className="flex items-center gap-2 text-sm font-semibold text-[var(--text-muted)]">
+          <Spinner className="size-4" />
+          {t(messageKey)}
+        </p>
+      </div>
+    </div>
+  );
+}