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/FolderBar.tsx

@@ -4,6 +4,7 @@ import { useRef, useState } from "react";
 
 import { FolderIcon, PlusIcon } from "@/components/icons";
 import { createFolder } from "@/lib/folders";
+import { useI18n } from "@/lib/i18n";
 import type { Folder } from "@/lib/schemas";
 import { cn } from "@/lib/utils";
 
@@ -32,6 +33,7 @@ export function FolderBar({
   const [draft, setDraft] = useState("");
   const [error, setError] = useState<string | null>(null);
   const inputRef = useRef<HTMLInputElement>(null);
+  const { t } = useI18n();
 
   const submit = () => {
     const result = createFolder(draft);
@@ -62,7 +64,7 @@ export function FolderBar({
           aria-pressed={selected === ALL_FOLDERS}
           className={pill(selected === ALL_FOLDERS)}
         >
-          All decks
+          {t("common.allDecks")}
         </button>
 
         {folders.map((folder) => (
@@ -97,8 +99,8 @@ export function FolderBar({
                   setError(null);
                 }
               }}
-              placeholder="Folder name"
-              aria-label="New folder name"
+              placeholder={t("common.folderName")}
+              aria-label={t("common.newFolderName")}
               maxLength={60}
               className="w-28 bg-transparent text-[13px] text-[var(--text)] outline-none placeholder:text-[var(--text-subtle)]"
             />
@@ -107,7 +109,7 @@ export function FolderBar({
               onClick={submit}
               className="btn-primary h-7 px-3 text-[12px]"
             >
-              Add
+              {t("common.add")}
             </button>
           </span>
         ) : (
@@ -117,7 +119,7 @@ export function FolderBar({
             className="inline-flex shrink-0 items-center gap-2 rounded-full px-3.5 py-1.5 text-[13px] font-medium text-[var(--text-subtle)] transition hover:bg-[var(--hover)] hover:text-[var(--text)]"
           >
             <PlusIcon className="size-4" />
-            New folder
+            {t("common.newFolder")}
           </button>
         )}
       </div>