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/TrueFalse.tsx

@@ -3,6 +3,7 @@
 import { motion, useMotionValue, useTransform } from "framer-motion";
 import { useState } from "react";
 
+import { useI18n } from "@/lib/i18n";
 import type { Activity } from "@/lib/schemas";
 import { Button } from "@/components/ui";
 
@@ -17,6 +18,7 @@ import {
 type TrueFalseActivity = Extract<Activity, { type: "true_false" }>;
 
 export function TrueFalse({ activity, onAnswered, onNext, isLast }: ActivityComponentProps<TrueFalseActivity>) {
+  const { t } = useI18n();
   const [result, setResult] = useState<boolean | null>(null);
   const x = useMotionValue(0);
   // The drag is elastic, so the card moves less than the pointer — these
@@ -37,7 +39,7 @@ export function TrueFalse({ activity, onAnswered, onNext, isLast }: ActivityComp
     <>
       <ActivityHeader type="true_false" topic={activity.topic} />
       <Hint>
-        {result === null ? "Swipe the card right for true, left for false — or tap a button." : "Nice swipe!"}
+        {result === null ? t("tf.hint") : t("tf.niceSwipe")}
       </Hint>
 
       <div className="relative">
@@ -55,10 +57,10 @@ export function TrueFalse({ activity, onAnswered, onNext, isLast }: ActivityComp
         >
           <PromptCard className="relative min-h-44 select-none">
             <motion.span style={{ opacity: trueOpacity }} className="absolute left-4 top-3 text-sm font-extrabold uppercase tracking-widest text-emerald-500">
-              ✓ true
+              {t("tf.trueLabel")}
             </motion.span>
             <motion.span style={{ opacity: falseOpacity }} className="absolute right-4 top-3 text-sm font-extrabold uppercase tracking-widest text-rose-500">
-              false ✕
+              {t("tf.falseLabel")}
             </motion.span>
             <p className="pt-6 text-center text-xl font-bold leading-snug text-frost-950 sm:text-2xl dark:text-frost-50">
               “{activity.statement}”
@@ -70,10 +72,10 @@ export function TrueFalse({ activity, onAnswered, onNext, isLast }: ActivityComp
       {result === null ? (
         <div className="grid grid-cols-2 gap-3">
           <Button variant="secondary" size="lg" onClick={() => answer(false)} className="border-rose-200 text-rose-700 dark:text-rose-200">
-            ✕ False
+            {t("tf.falseBtn")}
           </Button>
           <Button variant="success" size="lg" onClick={() => answer(true)}>
-            ✓ True
+            {t("tf.trueBtn")}
           </Button>
         </div>
       ) : (
@@ -85,7 +87,8 @@ export function TrueFalse({ activity, onAnswered, onNext, isLast }: ActivityComp
           onNext={onNext}
           detail={
             <span>
-              This statement is <span className="font-bold">{activity.answer ? "true" : "false"}</span>.
+              {t("tf.statementIs.before")}{" "}
+              <span className="font-bold">{activity.answer ? t("tf.true") : t("tf.false")}</span>.
             </span>
           }
         />