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/documents.ts

@@ -14,6 +14,7 @@
  * to the API routes. Photos uploaded as images keep working exactly as before.
  */
 
+import { t } from "./i18n";
 import { prepareImageFromBlob, type PreparedImage } from "./image";
 
 export const MAX_DOCUMENT_BYTES = 40 * 1024 * 1024; // 40 MB per file
@@ -114,10 +115,10 @@ export function isDocumentFile(file: File): boolean {
 
 export function describeDocumentError(file: File): string | null {
   if (!isDocumentFile(file)) {
-    return `"${file.name}" is not a supported document (use PDF, DOCX, PPTX, ODT/ODP or text).`;
+    return t("file.notDocument", { name: file.name });
   }
   if (file.size > MAX_DOCUMENT_BYTES) {
-    return `"${file.name}" is larger than 40 MB.`;
+    return t("file.documentTooLarge", { name: file.name });
   }
   return null;
 }
@@ -224,7 +225,7 @@ async function extractPdf(
 
   for (let pageNumber = 1; pageNumber <= totalPages; pageNumber++) {
     onProgress?.({
-      message: `Reading page ${pageNumber} of ${totalPages}…`,
+      message: t("doc.readingPage", { n: pageNumber, total: totalPages }),
       progress: pageNumber / totalPages,
     });
 
@@ -452,16 +453,16 @@ export async function extractDocument(
     case "pdf":
       return extractPdf(file, onProgress);
     case "docx":
-      onProgress?.({ message: "Reading the Word document…" });
+      onProgress?.({ message: t("doc.readingWord") });
       return extractDocx(file);
     case "pptx":
-      onProgress?.({ message: "Reading the slides…" });
+      onProgress?.({ message: t("doc.readingSlides") });
       return extractOoxmlPackage(file, "pptx");
     case "opendocument":
-      onProgress?.({ message: "Reading the OpenDocument file…" });
+      onProgress?.({ message: t("doc.readingOpen") });
       return extractOoxmlPackage(file, "opendocument");
     case "text":
-      onProgress?.({ message: "Reading the text file…" });
+      onProgress?.({ message: t("doc.readingText") });
       return extractPlainText(file);
   }
 }