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/offline/page.tsx

@@ -1,18 +1,18 @@
-import { HomeIcon } from "@/components/icons";
+"use client";
 
-export const metadata = { title: "Offline" };
+import { HomeIcon } from "@/components/icons";
+import { useI18n } from "@/lib/i18n";
 
 /** Shown by the service worker when a navigation can't reach the network. */
 export default function OfflinePage() {
+  const { t } = useI18n();
+
   return (
     <main className="frost-bg grid min-h-dvh place-items-center px-4">
       <div className="card relative max-w-sm space-y-3 p-6 text-center">
         <HomeIcon className="mx-auto size-8 text-frost-400" />
-        <h1 className="text-xl font-semibold text-frost-950 dark:text-frost-50">You&apos;re offline</h1>
-        <p className="text-sm text-frost-600 dark:text-frost-300">
-          Freezmo needs a connection for generating decks and grading answers. Your saved decks are
-          still in this browser — reconnect and reload.
-        </p>
+        <h1 className="text-xl font-semibold text-frost-950 dark:text-frost-50">{t("offline.title")}</h1>
+        <p className="text-sm text-frost-600 dark:text-frost-300">{t("offline.body")}</p>
       </div>
     </main>
   );