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/LibraryScreen.tsx

@@ -8,6 +8,7 @@ import { DeckCard } from "@/components/DeckCard";
 import { FolderBar } from "@/components/FolderBar";
 import { HomeworkCard } from "@/components/HomeworkCard";
 import { EllipsisIcon, FolderIcon, UploadIcon } from "@/components/icons";
+import { LinkLoadingScreen } from "@/components/LinkLoadingScreen";
 import { SegmentedPill } from "@/components/SegmentedPill";
 import { Button, EmptyState, ProgressBar } from "@/components/ui";
 import { createFolder, folderStore, renameFolder } from "@/lib/folders";
@@ -306,6 +307,7 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
         <div className="flex flex-wrap gap-2">
           <Link
             href="/review"
+            prefetch
             aria-disabled={due === 0}
             className={cn(
               "touch-target inline-flex h-10 items-center rounded-xl px-3.5 text-[13px] font-semibold transition",
@@ -313,9 +315,11 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
             )}
           >
             {t("review.title")} · {plural(due, "review.due")}
+            <LinkLoadingScreen messageKey="review.preparing" />
           </Link>
           <Link
             href="/review?mistakes=1"
+            prefetch
             aria-disabled={mistakes === 0}
             className={cn(
               "touch-target inline-flex h-10 items-center rounded-xl px-3.5 text-[13px] font-semibold transition",
@@ -323,6 +327,7 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
             )}
           >
             {t("review.mistakesTitle")} · {plural(mistakes, "review.mistakes")}
+            <LinkLoadingScreen messageKey="review.preparing" />
           </Link>
         </div>
       </section>