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/TabBar.tsx

@@ -4,6 +4,7 @@ import Link from "next/link";
 import { usePathname } from "next/navigation";
 
 import { GearIcon, HomeIcon, PlusIcon } from "@/components/icons";
+import { useI18n } from "@/lib/i18n";
 import { openSettings } from "@/lib/ui-events";
 import { cn } from "@/lib/utils";
 
@@ -14,6 +15,7 @@ import { cn } from "@/lib/utils";
  */
 export function TabBar() {
   const pathname = usePathname();
+  const { t } = useI18n();
 
   const item = "flex flex-1 flex-col items-center justify-center gap-1 py-2 text-[11px] font-medium transition-colors";
   const active = "text-[var(--text)]";
@@ -21,12 +23,12 @@ export function TabBar() {
 
   return (
     <nav
-      aria-label="Main"
+      aria-label={t("tabbar.main")}
       className="tabbar fixed inset-x-0 bottom-0 z-40 flex items-stretch pb-safe sm:hidden"
     >
       <Link href="/" aria-current={pathname === "/" ? "page" : undefined} className={cn(item, pathname === "/" ? active : idle)}>
         <HomeIcon className="size-6" />
-        Library
+        {t("common.library")}
       </Link>
       <Link
         href="/create"
@@ -36,11 +38,11 @@ export function TabBar() {
         <span className="grad-ice grid size-6 place-items-center rounded-full text-[#06111f]">
           <PlusIcon className="size-4" />
         </span>
-        New deck
+        {t("common.newDeck")}
       </Link>
       <button type="button" onClick={openSettings} className={cn(item, idle)}>
         <GearIcon className="size-6" />
-        Settings
+        {t("common.settings")}
       </button>
     </nav>
   );