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).

components/activities/MultipleChoice.tsx

@@ -2,6 +2,7 @@
 
 import { useEffect, useState } from "react";
 
+import { useI18n } from "@/lib/i18n";
 import type { Activity } from "@/lib/schemas";
 
 import {
@@ -23,6 +24,7 @@ export function MultipleChoice({
   onNext,
   isLast,
 }: ActivityComponentProps<MultipleChoiceActivity>) {
+  const { t } = useI18n();
   const [selected, setSelected] = useState<number | null>(null);
   const answered = selected !== null;
 
@@ -62,7 +64,7 @@ export function MultipleChoice({
         <QuestionText>{activity.question}</QuestionText>
       </PromptCard>
 
-      <div className="flex flex-col gap-2.5" role="group" aria-label="Answer options">
+      <div className="flex flex-col gap-2.5" role="group" aria-label={t("activity.answerOptions")}>
         {activity.options.map((option, index) => (
           <OptionTile
             key={option + index}
@@ -76,7 +78,7 @@ export function MultipleChoice({
         ))}
       </div>
 
-      {!answered ? <Hint>Tip: press 1–4 on a keyboard.</Hint> : null}
+      {!answered ? <Hint>{t("mc.tipKeys")}</Hint> : null}
 
       {answered ? (
         <FeedbackPanel
@@ -88,7 +90,7 @@ export function MultipleChoice({
           detail={
             selected === activity.correctIndex ? null : (
               <>
-                Correct answer:{" "}
+                {t("activity.correctAnswer")}{" "}
                 <span className="font-bold">{activity.options[activity.correctIndex]}</span>
               </>
             )