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/generate/route.ts

@@ -1,10 +1,11 @@
 import { z } from "zod";
 
 import { extractNotes, generateActivities, notesToText, type PipelineStage } from "@/lib/ai";
-import { AUTH_ERROR_MESSAGES, authErrorKind, getServerConfig, resolveAuth, resolveModelName } from "@/lib/config";
+import { authErrorMessage, authErrorKind, getServerConfig, resolveAuth, resolveModelName } 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";
 import { difficultySchema } from "@/lib/schemas";
 import { uid } from "@/lib/utils";
 import { xpForActivity } from "@/lib/xp";
@@ -48,6 +49,7 @@ function encoder() {
 const MAX_BODY_BYTES = 33554432;
 
 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);
@@ -57,12 +59,12 @@ export async function POST(request: Request) {
 
   // Cheap requests are rejected before any work happens.
   const limit = rateLimit(`generate:${clientKey(request)}`, config.rateLimits.generate, 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 },
     );
   }
@@ -71,17 +73,19 @@ 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 request: ${parsed.error.issues
-          .slice(0, 3)
-          .map((issue) => `${issue.path.join(".") || "body"} ${issue.message}`)
-          .join("; ")}`,
+        error: translate(locale, "server.invalidRequest", {
+          detail: parsed.error.issues
+            .slice(0, 3)
+            .map((issue) => `${issue.path.join(".") || "body"} ${issue.message}`)
+            .join("; "),
+        }),
       },
       { status: 400 },
     );
@@ -92,7 +96,7 @@ export async function POST(request: Request) {
 
   if (imageList.length === 0 && !text?.trim()) {
     return Response.json(
-      { error: "Add at least one photo of your notes or paste some text to build a deck." },
+      { error: translate(locale, "server.needContent") },
       { status: 400 },
     );
   }
@@ -187,7 +191,7 @@ export async function POST(request: Request) {
           },
         });
       } catch (error) {
-        const message = friendlyErrorMessage(error, auth.auth.baseUrl);
+        const message = friendlyErrorMessage(error, auth.auth.baseUrl, locale);
         // Never log the request body: it may contain the user's notes.
         console.error("[api/generate]", error instanceof OllamaError ? `${error.kind}: ${error.message}` : error);
         send({