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/folders.ts

@@ -8,6 +8,7 @@
  * never persisted — it is simply every deck with no `folderId`.
  */
 
+import { t } from "./i18n";
 import { folderSchema, UNORGANIZED_ID, type Folder } from "./schemas";
 import { uid } from "./utils";
 
@@ -99,8 +100,8 @@ export function getFolder(id: string | undefined): Folder | undefined {
 }
 
 export function folderName(id: string | undefined): string {
-  if (!id || id === UNORGANIZED_ID) return "Unorganized";
-  return getFolder(id)?.name ?? "Unorganized";
+  if (!id || id === UNORGANIZED_ID) return t("common.unorganized");
+  return getFolder(id)?.name ?? t("common.unorganized");
 }
 
 export function findFolderByName(name: string): Folder | undefined {
@@ -111,8 +112,8 @@ export function findFolderByName(name: string): Folder | undefined {
 /** Case-insensitive uniqueness; returns the existing folder when it matches. */
 export function createFolder(name: string): { folder: Folder } | { error: string } {
   const trimmed = name.trim().replace(/\s+/g, " ");
-  if (!trimmed) return { error: "Give the folder a name." };
-  if (trimmed.length > 60) return { error: "That name is a bit long (60 characters max)." };
+  if (!trimmed) return { error: t("folder.nameRequired") };
+  if (trimmed.length > 60) return { error: t("folder.nameTooLong") };
 
   const existing = findFolderByName(trimmed);
   if (existing) return { folder: existing };
@@ -125,11 +126,11 @@ export function createFolder(name: string): { folder: Folder } | { error: string
 
 export function renameFolder(id: string, name: string): { ok: true } | { error: string } {
   const trimmed = name.trim().replace(/\s+/g, " ");
-  if (!trimmed) return { error: "Give the folder a name." };
+  if (!trimmed) return { error: t("folder.nameRequired") };
   const clash = folderStore
     .getSnapshot()
     .find((folder) => folder.id !== id && folder.name.trim().toLowerCase() === trimmed.toLowerCase());
-  if (clash) return { error: "Another folder already has that name." };
+  if (clash) return { error: t("folder.nameTaken") };
 
   ensureLoaded();
   write(folderStore.getSnapshot().map((folder) => (folder.id === id ? { ...folder, name: trimmed } : folder)));