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).

app/api/grade/route.ts

@@ -1,10 +1,11 @@
 import { z } from "zod";
 
 import { gradeShortAnswer } from "@/lib/ai";
-import { AUTH_ERROR_MESSAGES, authErrorKind, getServerConfig, resolveAuth } from "@/lib/config";
+import { authErrorMessage, authErrorKind, getServerConfig, resolveAuth } from "@/lib/config";
 import { friendlyErrorMessage, OllamaError } from "@/lib/ollama";
-import { rejectCrossOrigin, rejectOversizedBody } from "@/lib/http-guard";
+import { rejectCrossOrigin, rejectOversizedBody, requestLocale } from "@/lib/http-guard";
 import { clientKey, rateLimit, rateLimitResponse } from "@/lib/rate-limit";
+import { translate } from "@/lib/i18n-dict";
 
 export const runtime = "nodejs";
 export const dynamic = "force-dynamic";
@@ -22,6 +23,7 @@ const requestSchema = z.object({
 const MAX_BODY_BYTES = 65536;
 
 export async function POST(request: Request) {
+  const locale = requestLocale(request);
   const crossOrigin = rejectCrossOrigin(request);
   if (crossOrigin) return crossOrigin;
   const oversized = rejectOversizedBody(request, MAX_BODY_BYTES);
@@ -30,12 +32,12 @@ export async function POST(request: Request) {
   const config = getServerConfig();
 
   const limit = rateLimit(`grade:${clientKey(request)}`, config.rateLimits.grade, config.rateLimits.windowMs);
-  if (!limit.allowed) return rateLimitResponse(limit);
+  if (!limit.allowed) return rateLimitResponse(limit, locale);
 
   const auth = resolveAuth(config, request);
   if ("error" in auth) {
     return Response.json(
-      { error: AUTH_ERROR_MESSAGES[auth.error], kind: authErrorKind(auth.error) },
+      { error: authErrorMessage(locale, auth.error), kind: authErrorKind(auth.error) },
       { status: 401 },
     );
   }
@@ -44,12 +46,12 @@ export async function POST(request: Request) {
   try {
     payload = await request.json();
   } catch {
-    return Response.json({ error: "Request body was not valid JSON." }, { status: 400 });
+    return Response.json({ error: translate(locale, "server.bodyNotJson") }, { status: 400 });
   }
 
   const parsed = requestSchema.safeParse(payload);
   if (!parsed.success) {
-    return Response.json({ error: "Invalid grading request." }, { status: 400 });
+    return Response.json({ error: translate(locale, "server.invalidGrade") }, { status: 400 });
   }
 
   try {
@@ -64,7 +66,10 @@ export async function POST(request: Request) {
   } catch (error) {
     console.error("[api/grade]", error instanceof OllamaError ? `${error.kind}: ${error.message}` : error);
     return Response.json(
-      { error: friendlyErrorMessage(error, auth.auth.baseUrl), kind: error instanceof OllamaError ? error.kind : "unknown" },
+      {
+        error: friendlyErrorMessage(error, auth.auth.baseUrl, locale),
+        kind: error instanceof OllamaError ? error.kind : "unknown",
+      },
       { status: 502 },
     );
   }