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/Flashcard.tsx

@@ -3,6 +3,7 @@
 import { motion } from "framer-motion";
 import { useState } from "react";
 
+import { useI18n } from "@/lib/i18n";
 import type { Activity } from "@/lib/schemas";
 import { Button } from "@/components/ui";
 
@@ -18,6 +19,7 @@ import {
 type FlashcardActivity = Extract<Activity, { type: "flashcard" }>;
 
 export function Flashcard({ activity, onAnswered, onNext, isLast }: ActivityComponentProps<FlashcardActivity>) {
+  const { t } = useI18n();
   const [flipped, setFlipped] = useState(false);
   const [result, setResult] = useState<boolean | null>(null);
 
@@ -30,13 +32,13 @@ export function Flashcard({ activity, onAnswered, onNext, isLast }: ActivityComp
   return (
     <>
       <ActivityHeader type="flashcard" topic={activity.topic} />
-      <Hint>{flipped ? "Did you know it before flipping?" : "Tap the card to flip it."}</Hint>
+      <Hint>{flipped ? t("flashcard.didYouKnow") : t("flashcard.tapToFlip")}</Hint>
 
       <button
         type="button"
         onClick={() => setFlipped((value) => !value)}
         className="group relative block w-full text-left [perspective:1400px]"
-        aria-label={flipped ? "Flip back to the question" : "Reveal the answer"}
+        aria-label={flipped ? t("flashcard.flipBack") : t("flashcard.reveal")}
       >
         <motion.div
           animate={{ rotateX: flipped ? 180 : 0 }}
@@ -45,18 +47,18 @@ export function Flashcard({ activity, onAnswered, onNext, isLast }: ActivityComp
         >
           <PromptCard className="absolute inset-0 flex flex-col justify-center gap-3 group-hover:border-frost-300 [backface-visibility:hidden]">
             <span className="text-xs font-bold uppercase tracking-widest text-frost-600 dark:text-frost-200">
-              Question
+              {t("flashcard.question")}
             </span>
             <QuestionText>{activity.front}</QuestionText>
             <span className="mt-2 text-xs font-semibold text-frost-400 dark:text-frost-400">
-              tap to flip ↻
+              {t("flashcard.tapFlip")}
             </span>
           </PromptCard>
 
           <div className="[transform:rotateX(180deg)] [backface-visibility:hidden]">
             <div className="card flex min-h-56 flex-col justify-center gap-3 px-5 py-6 sm:px-6">
               <span className="text-xs font-bold uppercase tracking-widest text-frost-600 dark:text-frost-200">
-                Answer
+                {t("flashcard.answer")}
               </span>
               <p className="text-lg font-semibold leading-relaxed text-frost-950 dark:text-frost-50">
                 {activity.back}
@@ -69,10 +71,10 @@ export function Flashcard({ activity, onAnswered, onNext, isLast }: ActivityComp
       {flipped && result === null ? (
         <div className="grid grid-cols-2 gap-3">
           <Button variant="secondary" size="lg" onClick={() => settle(false)}>
-            😅 Not yet
+            {t("flashcard.notYet")}
           </Button>
           <Button variant="success" size="lg" onClick={() => settle(true)}>
-            🧊 I knew it
+            {t("flashcard.knewIt")}
           </Button>
         </div>
       ) : null}
@@ -83,7 +85,7 @@ export function Flashcard({ activity, onAnswered, onNext, isLast }: ActivityComp
           explanation={activity.explanation}
           xp={activity.xp}
           isLast={isLast}
-          headline={result ? "Frozen solid!" : "Keep it on the pile"}
+          headline={result ? t("flashcard.frozenSolid") : t("flashcard.keepPile")}
           onNext={onNext}
         />
       ) : null}