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/Matching.tsx

@@ -2,6 +2,7 @@
 
 import { useMemo, useState } from "react";
 
+import { useI18n } from "@/lib/i18n";
 import type { Activity } from "@/lib/schemas";
 import { cn, shuffle } from "@/lib/utils";
 
@@ -16,6 +17,7 @@ import {
 type MatchingActivity = Extract<Activity, { type: "matching" }>;
 
 export function Matching({ activity, onAnswered, onNext, isLast }: ActivityComponentProps<MatchingActivity>) {
+  const { t, plural } = useI18n();
   const terms = useMemo(
     () => shuffle(activity.pairs.map((pair, index) => ({ index, text: pair.term }))),
     [activity],
@@ -60,7 +62,7 @@ export function Matching({ activity, onAnswered, onNext, isLast }: ActivityCompo
         topic={activity.topic}
         extra={
           <span className="rounded-full bg-[var(--surface-strong)] px-3 py-1 text-xs font-bold text-frost-600 dark:text-frost-200">
-            {matched.length}/{total} matched
+            {t("matching.matched", { n: matched.length, total })}
           </span>
         }
       />
@@ -68,10 +70,10 @@ export function Matching({ activity, onAnswered, onNext, isLast }: ActivityCompo
         <p className="text-lg font-bold text-frost-950 dark:text-frost-50">{activity.instruction}</p>
         <Hint>
           {done
-            ? "All matched!"
+            ? t("matching.allMatched")
             : selectedTerm === null
-              ? "Tap a term on the left, then its match on the right."
-              : "Now tap the matching definition →"}
+              ? t("matching.tapTerm")
+              : t("matching.tapDef")}
         </Hint>
       </PromptCard>
 
@@ -136,12 +138,10 @@ export function Matching({ activity, onAnswered, onNext, isLast }: ActivityCompo
           xp={activity.xp}
           isLast={isLast}
           onNext={onNext}
-          headline={misses === 0 ? "Perfect match!" : "Matched — with a few slips"}
+          headline={misses === 0 ? t("matching.perfect") : t("matching.slips")}
           detail={
             <span>
-              {misses === 0
-                ? "You matched everything first try."
-                : `You made ${misses} wrong ${misses === 1 ? "attempt" : "attempts"}.`}
+              {misses === 0 ? t("matching.firstTry") : plural(misses, "matching.wrongAttempts")}
             </span>
           }
         />