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/ollama.ts

@@ -9,6 +9,7 @@
  * This module must only ever be imported from server code (route handlers).
  */
 
+import { DEFAULT_LOCALE, translate, type Locale } from "./i18n-dict";
 import { getServerConfig, resolveModelName, type OllamaAuth } from "./config";
 
 export type OllamaMessage = {
@@ -41,25 +42,31 @@ export class OllamaError extends Error {
   }
 }
 
-export function friendlyErrorMessage(error: unknown, baseUrl?: string): string {
+export function friendlyErrorMessage(
+  error: unknown,
+  baseUrl?: string,
+  locale: Locale = DEFAULT_LOCALE,
+): string {
   if (error instanceof OllamaError) {
     switch (error.kind) {
       case "unreachable":
-        return `Ollama isn't reachable at ${baseUrl ?? "the configured URL"}. Check your settings and make sure the daemon is running.`;
+        return translate(locale, "server.ollamaUnreachable", {
+          url: baseUrl ?? "the configured URL",
+        });
       case "timeout":
-        return "Ollama took too long to answer. Try fewer/smaller images, a faster model, or retry.";
+        return translate(locale, "server.ollamaTimeout");
       case "auth":
-        return "Ollama rejected the request. If you are using https://ollama.com, check OLLAMA_API_KEY (create one at ollama.com/settings/keys).";
+        return translate(locale, "server.ollamaAuth");
       case "no_credits":
         return error.message;
       case "model_missing":
         return error.message;
       default:
-        return error.message || "Ollama returned an unexpected response.";
+        return error.message || translate(locale, "server.ollamaUnexpected");
     }
   }
   if (error instanceof Error) return error.message;
-  return "Something went wrong while talking to Ollama.";
+  return translate(locale, "server.ollamaGeneric");
 }
 
 type ChatOptions = {