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/Ordering.tsx

@@ -3,6 +3,7 @@
 import { Reorder } from "framer-motion";
 import { useMemo, useState } from "react";
 
+import { useI18n } from "@/lib/i18n";
 import type { Activity } from "@/lib/schemas";
 import { cn, shuffle } from "@/lib/utils";
 import { Button } from "@/components/ui";
@@ -18,6 +19,7 @@ import {
 type OrderingActivity = Extract<Activity, { type: "ordering" }>;
 
 export function Ordering({ activity, onAnswered, onNext, isLast }: ActivityComponentProps<OrderingActivity>) {
+  const { t } = useI18n();
   const initial = useMemo(() => shuffle(activity.items), [activity]);
   const [items, setItems] = useState<string[]>(initial);
   const [checked, setChecked] = useState(false);
@@ -48,7 +50,7 @@ export function Ordering({ activity, onAnswered, onNext, isLast }: ActivityCompo
       <PromptCard className="py-4">
         <p className="text-lg font-bold text-frost-950 dark:text-frost-50">{activity.instruction}</p>
         <Hint>
-          {checked ? "Here is the correct order:" : "Drag the cards, or use ▲▼, then check your order."}
+          {checked ? t("ordering.correctOrder") : t("ordering.hint")}
         </Hint>
       </PromptCard>
 
@@ -89,7 +91,7 @@ export function Ordering({ activity, onAnswered, onNext, isLast }: ActivityCompo
               <span className="flex flex-col">
                 <button
                   type="button"
-                  aria-label={`Move "${item}" up`}
+                  aria-label={t("ordering.moveUp", { item })}
                   onClick={() => move(index, -1)}
                   disabled={index === 0}
                   className="rounded px-1.5 text-xs text-frost-700 transition hover:bg-frost-100 disabled:opacity-30 dark:text-frost-100 dark:hover:bg-white/10"
@@ -98,7 +100,7 @@ export function Ordering({ activity, onAnswered, onNext, isLast }: ActivityCompo
                 </button>
                 <button
                   type="button"
-                  aria-label={`Move "${item}" down`}
+                  aria-label={t("ordering.moveDown", { item })}
                   onClick={() => move(index, 1)}
                   disabled={index === items.length - 1}
                   className="rounded px-1.5 text-xs text-frost-700 transition hover:bg-frost-100 disabled:opacity-30 dark:text-frost-100 dark:hover:bg-white/10"
@@ -116,7 +118,7 @@ export function Ordering({ activity, onAnswered, onNext, isLast }: ActivityCompo
 
       {!checked ? (
         <Button size="lg" onClick={check} disabled={items.length < 2}>
-          Check order
+          {t("ordering.check")}
         </Button>
       ) : (
         <FeedbackPanel