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/AppHeader.tsx

@@ -6,6 +6,7 @@ import { useEffect, useState } from "react";
 import { GearIcon, MenuIcon } from "@/components/icons";
 import { SettingsDialog } from "@/components/SettingsDialog";
 import { ThemeToggle } from "@/components/ThemeToggle";
+import { useI18n } from "@/lib/i18n";
 import { sidebarOpen } from "@/lib/nav";
 import { OPEN_SETTINGS_EVENT } from "@/lib/ui-events";
 import { cn } from "@/lib/utils";
@@ -16,6 +17,7 @@ import { cn } from "@/lib/utils";
  * for navigation.
  */
 export function AppHeader({ className, compact = false }: { className?: string; compact?: boolean }) {
+  const { t } = useI18n();
   const [settingsOpen, setSettingsOpen] = useState(false);
 
   useEffect(() => {
@@ -41,12 +43,12 @@ export function AppHeader({ className, compact = false }: { className?: string;
           <button
             type="button"
             onClick={() => sidebarOpen.toggle()}
-            aria-label="Open menu"
+            aria-label={t("header.openMenu")}
             className="mr-1 grid size-9 shrink-0 place-items-center rounded-full text-[var(--text-muted)] transition hover:bg-[var(--hover)] hover:text-[var(--text)]"
           >
             <MenuIcon className="size-5" />
           </button>
-          <Link href="/" className="group flex items-center gap-2.5 rounded-xl px-1 py-1" aria-label="Freezmo home">
+          <Link href="/" className="group flex items-center gap-2.5 rounded-xl px-1 py-1" aria-label={t("header.home")}>
           {/* eslint-disable-next-line @next/next/no-img-element -- static app icon, no optimisation needed */}
           <img
             src="/icons/icon-192.png"
@@ -60,7 +62,7 @@ export function AppHeader({ className, compact = false }: { className?: string;
               </span>
               {!compact ? (
                 <span className="hidden text-[13px] text-[var(--text-subtle)] sm:inline">
-                  study feed
+                  {t("header.tagline")}
                 </span>
               ) : null}
             </span>
@@ -70,8 +72,8 @@ export function AppHeader({ className, compact = false }: { className?: string;
             <button
               type="button"
               onClick={() => setSettingsOpen(true)}
-              aria-label="Open settings and Ollama status"
-              title="Settings"
+              aria-label={t("header.openSettings")}
+              title={t("common.settings")}
               className="inline-flex size-9 items-center justify-center rounded-full text-[var(--text-muted)] transition hover:bg-[var(--hover)] hover:text-[var(--text)]"
             >
               <GearIcon className="size-[18px]" />