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/FillBlank.tsx

@@ -2,6 +2,7 @@
 
 import { useEffect, useRef, useState } from "react";
 
+import { useI18n } from "@/lib/i18n";
 import type { Activity } from "@/lib/schemas";
 import { cn } from "@/lib/utils";
 import { Button } from "@/components/ui";
@@ -67,6 +68,7 @@ export function isFillBlankCorrect(userValue: string, answer: string): boolean {
 }
 
 export function FillBlank({ activity, active, onAnswered, onNext, isLast }: ActivityComponentProps<FillBlankActivity>) {
+  const { t } = useI18n();
   const [value, setValue] = useState("");
   const [chosen, setChosen] = useState<string | null>(null);
   const [result, setResult] = useState<boolean | null>(null);
@@ -121,8 +123,8 @@ export function FillBlank({ activity, active, onAnswered, onNext, isLast }: Acti
                 event.stopPropagation();
               }}
               disabled={result !== null}
-              aria-label="Type the missing word"
-              placeholder="type it"
+              aria-label={t("fill.typeMissing")}
+              placeholder={t("fill.placeholder")}
               autoComplete="off"
               autoCapitalize="off"
               spellCheck={false}
@@ -142,7 +144,7 @@ export function FillBlank({ activity, active, onAnswered, onNext, isLast }: Acti
       </PromptCard>
 
       {activity.options ? (
-        <div className="grid grid-cols-2 gap-2.5" role="group" aria-label="Word bank">
+        <div className="grid grid-cols-2 gap-2.5" role="group" aria-label={t("activity.wordBank")}>
           {activity.options.map((option) => {
             const state =
               result === null
@@ -168,13 +170,13 @@ export function FillBlank({ activity, active, onAnswered, onNext, isLast }: Acti
       ) : result === null ? (
         <div className="flex gap-2">
           <Button size="lg" onClick={() => submit(value)} disabled={!value.trim()} className="flex-1">
-            Check answer
+            {t("fill.check")}
           </Button>
         </div>
       ) : null}
 
       {result === null && !activity.options ? (
-        <Hint>Press Enter to check. Small typos are forgiven.</Hint>
+        <Hint>{t("fill.hint")}</Hint>
       ) : null}
 
       {result !== null ? (
@@ -186,11 +188,11 @@ export function FillBlank({ activity, active, onAnswered, onNext, isLast }: Acti
           onNext={onNext}
           detail={
             <span>
-              Expected: <span className="font-bold">{activity.answer}</span>
+              {t("fill.expected")} <span className="font-bold">{activity.answer}</span>
               {!result && chosen ? (
                 <>
                   {" "}
-                  — you wrote <span className="italic">{chosen}</span>
+                  {t("fill.youWrote")} <span className="italic">{chosen}</span>
                 </>
               ) : null}
             </span>