Add an English/Czech interface language switch

English stays the default; Czech is a full translation. Only the app chrome
is translated - notes and generated decks keep their own language, which is a
core feature, not a setting.

i18n
- lib/i18n-dict.ts: flat en/cs dictionaries (441 keys each), translate() and
  translatePlural() with real Czech plural rules (1 sada / 2-4 sady / 5+ sad).
  English is the default and the fallback for a missing key.
- lib/i18n.ts: browser store (freezmo.locale.v1), useI18n() hook, and the
  x-freezmo-locale header. The hook's t/plural are bound to the locale React
  is rendering with, so the first client render matches the server HTML and
  then re-renders in the stored language; module-level t is only for event
  handlers and non-React helpers.
- <html lang> follows the choice (inline script + setLocale).

UI
- Settings -> Language segmented control (English / Cestina), plus an EN/CS
  button on the key gate for first run.
- Every screen translated: sidebar, header, tab bar, library, deck cards,
  folder bar, create flow, all seven activities, study chrome, results,
  settings + health panel, API-key gate and help, install sheet, offline and
  the deck-not-found page.
- Plural-aware counts everywhere (decks, folders, activities, runs, photos,
  documents, pages, words, wrong attempts).

Server
- API routes and guards (generate, grade, health, http-guard, rate-limit,
  ollama error mapping) answer in the request's language, so error cards and
  the health panel never fall back to English mid-sentence.

Fixes
- A hydration mismatch when a non-default locale was stored silently dropped
  the .dark class after hydration; render-time t() is now bound to the
  hook's locale (UploadScreen.describeExtraction, ResultsScreen.answerFor,
  DeckCard's folder fallback).
- Bumped the service worker cache to freezmo-v4 for this deploy.

Tests: i18n 27/27 (gate toggle, persistence, html lang, Czech plurals,
settings switch) and a rebuilt regression suite 72/72 (English default,
library, create, all 7 activity types, results, settings, install sheet,
offline, PWA, security, Czech smoke).

app/deck/[id]/page.tsx

@@ -7,6 +7,7 @@ import { useEffect, useState } from "react";
 import { ApiKeyGate } from "@/components/ApiKeyGate";
 import { StudyFeed } from "@/components/StudyFeed";
 import { Button, Spinner } from "@/components/ui";
+import { useI18n } from "@/lib/i18n";
 import type { Deck } from "@/lib/schemas";
 import { getDeck } from "@/lib/storage";
 
@@ -15,6 +16,7 @@ export default function DeckPage() {
   const id = typeof params.id === "string" ? params.id : "";
   const [deck, setDeck] = useState<Deck | null>(null);
   const [loaded, setLoaded] = useState(false);
+  const { t } = useI18n();
 
   useEffect(() => {
     if (!id) {
@@ -29,7 +31,7 @@ export default function DeckPage() {
     return (
       <main className="frost-bg grid min-h-dvh place-items-center">
         <div className="relative flex items-center gap-3 text-sm font-semibold text-frost-800 dark:text-frost-100">
-          <Spinner className="size-5" /> Loading your deck…
+          <Spinner className="size-5" /> {t("deckPage.loading")}
         </div>
       </main>
     );
@@ -42,20 +44,19 @@ export default function DeckPage() {
           <p className="text-4xl" aria-hidden>
             🧊
           </p>
-          <h1 className="text-xl font-extrabold text-frost-950 dark:text-frost-50">Deck not found</h1>
+          <h1 className="text-xl font-extrabold text-frost-950 dark:text-frost-50">{t("deckPage.notFound")}</h1>
           <p className="text-sm text-frost-600 dark:text-frost-300">
-            This deck isn&apos;t in this browser&apos;s library. It may have been deleted, or it was
-            created in a different browser profile.
+            {t("deckPage.notFoundBody")}
           </p>
           <div className="flex justify-center gap-2 pt-1">
             <Link
               href="/"
               className="inline-flex items-center gap-2 rounded-2xl bg-accent-600 px-4 py-2.5 text-sm font-semibold text-white transition-colors hover:bg-accent-700"
             >
-              Back to library
+              {t("deckPage.back")}
             </Link>
             <Link href="/create">
-              <Button variant="secondary">Create a deck</Button>
+              <Button variant="secondary">{t("deckPage.create")}</Button>
             </Link>
           </div>
         </div>