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/ThemeToggle.tsx

@@ -2,6 +2,7 @@
 
 import { useEffect, useState } from "react";
 
+import { useI18n } from "@/lib/i18n";
 import { cn } from "@/lib/utils";
 import { MoonIcon, SunIcon } from "@/components/icons";
 
@@ -33,14 +34,15 @@ export function useTheme() {
 
 export function ThemeToggle({ className }: { className?: string }) {
   const { theme, toggle } = useTheme();
+  const { t } = useI18n();
   const dark = theme === "dark";
 
   return (
     <button
       type="button"
       onClick={toggle}
-      aria-label={dark ? "Switch to light mode" : "Switch to dark mode"}
-      title={dark ? "Light mode" : "Dark mode"}
+      aria-label={dark ? t("common.switchToLight") : t("common.switchToDark")}
+      title={dark ? t("common.lightMode") : t("common.darkMode")}
       className={cn(
         "inline-flex size-9 items-center justify-center rounded-full text-[var(--text-muted)] transition hover:bg-[var(--hover)] hover:text-[var(--text)]",
         className,