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.

app/exam/loading.tsx

@@ -0,0 +1,6 @@
+import { RouteLoading } from "@/components/RouteLoading";
+
+/** Shown the instant the exam route starts loading. */
+export default function ExamLoading() {
+  return <RouteLoading messageKey="exam.preparing" />;
+}

app/review/loading.tsx

@@ -0,0 +1,6 @@
+import { RouteLoading } from "@/components/RouteLoading";
+
+/** Shown the instant the review route starts loading. */
+export default function ReviewLoading() {
+  return <RouteLoading messageKey="review.preparing" />;
+}

app/stats/loading.tsx

@@ -0,0 +1,6 @@
+import { RouteLoading } from "@/components/RouteLoading";
+
+/** Shown the instant the progress route starts loading. */
+export default function StatsLoading() {
+  return <RouteLoading messageKey="stats.preparing" />;
+}