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/i18n-dict.ts

@@ -807,6 +807,7 @@ const en: Record<string, string> = {
   "review.seeResults": "See results",
   "review.finishFirst": "Finish the last activity to see your results.",
   "review.results": "Results",
+  "review.preparing": "Preparing your review…",
   "review.due.one": "{n} due",
   "review.due.few": "{n} due",
   "review.due.many": "{n} due",
@@ -814,6 +815,7 @@ const en: Record<string, string> = {
   "review.mistakes.few": "{n} mistakes",
   "review.mistakes.many": "{n} mistakes",
   "exam.title": "Exam mode",
+  "exam.preparing": "Preparing exam mode…",
   "exam.subtitle": "Pick one or more decks, set a timer, no hints — see how you'd do.",
   "exam.pickDecks": "Decks",
   "exam.noDecks": "Create a deck first.",
@@ -827,6 +829,7 @@ const en: Record<string, string> = {
   "exam.studyMistakes": "Study mistakes",
   "exam.timeLeft": "{time} left",
   "stats.title": "Your progress",
+  "stats.preparing": "Preparing your progress…",
   "stats.subtitle": "Streaks, accuracy and the topics that deserve another pass.",
   "stats.streak": "Day streak",
   "stats.today": "Today",
@@ -1643,13 +1646,15 @@ const cs: Record<string, string> = {
   "review.seeResults": "Zobrazit výsledky",
   "review.finishFirst": "Dokonči poslední aktivitu a uvidíš výsledky.",
   "review.results": "Výsledky",
+  "review.preparing": "Připravuji opakování…",
   "review.due.one": "{n} k opakování",
   "review.due.few": "{n} k opakování",
   "review.due.many": "{n} k opakování",
   "review.mistakes.one": "{n} chyba",
   "review.mistakes.few": "{n} chyby",
   "review.mistakes.many": "{n} chyb",
-  "exam.title": "Zkouškové režim",
+  "exam.title": "Zkouškový režim",
+  "exam.preparing": "Připravuji zkouškový režim…",
   "exam.subtitle": "Vyber sady, nastav časovač, žádné nápovědy — uvidíš, jak bys dopadl.",
   "exam.pickDecks": "Sady",
   "exam.noDecks": "Nejdřív si vytvoř sadu.",
@@ -1663,6 +1668,7 @@ const cs: Record<string, string> = {
   "exam.studyMistakes": "Procvičit chyby",
   "exam.timeLeft": "zbývá {time}",
   "stats.title": "Tvůj pokrok",
+  "stats.preparing": "Připravuji pokrok…",
   "stats.subtitle": "Série, úspěšnost a témata, která si zaslouží další kolo.",
   "stats.streak": "Série dní",
   "stats.today": "Dnes",