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/LinkLoadingScreen.tsx

@@ -0,0 +1,23 @@
+"use client";
+
+import { useLinkStatus } from "next/link";
+
+import { RouteLoading } from "@/components/RouteLoading";
+
+/**
+ * A full-screen loading screen that appears the instant the surrounding
+ * `<Link>` starts navigating — before any response arrives — so a slow route
+ * never looks like a dead tap. Renders nothing while idle and disappears with
+ * the page that owns it.
+ */
+export function LinkLoadingScreen({ messageKey }: { messageKey: string }) {
+  const { pending } = useLinkStatus();
+
+  if (!pending) return null;
+
+  return (
+    <div className="pointer-events-none fixed inset-0 z-50">
+      <RouteLoading messageKey={messageKey} />
+    </div>
+  );
+}