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).

lib/storage.ts

@@ -5,6 +5,7 @@
  * Exposes a tiny observable store so every screen stays in sync.
  */
 
+import { t } from "./i18n";
 import { deckSchema, UNORGANIZED_ID, type Activity, type Deck } from "./schemas";
 import { createFolder, findFolderByName, getFolder, removeFolder } from "./folders";
 import { uid } from "./utils";
@@ -224,7 +225,7 @@ export function importDeck(raw: string): { deck: Deck } | { error: string } {
   try {
     parsed = JSON.parse(raw);
   } catch {
-    return { error: "That file is not valid JSON." };
+    return { error: t("import.notJson") };
   }
 
   const bundle = parsed && typeof parsed === "object" && "deck" in parsed ? (parsed as { folder?: unknown }) : null;
@@ -232,7 +233,7 @@ export function importDeck(raw: string): { deck: Deck } | { error: string } {
 
   const result = deckSchema.safeParse(candidate);
   if (!result.success) {
-    return { error: "That file doesn't look like a Freezmo deck." };
+    return { error: t("import.notDeck") };
   }
 
   // Recreate the exported folder (or reuse an existing one with the same name).