fix(ai): warn that upscale and background removal are slow without a GPU (#605)

Add a one-line notice to the upscale and background-removal settings that
heavy AI runs much slower without a GPU and a large image can take minutes,
so a CPU-only self-hoster isn't caught off guard by a slow run. Translated
into all 21 locales.

Refs #591

tests/unit/web/ai-cpu-note.test.tsx

@@ -0,0 +1,30 @@
+// @vitest-environment jsdom
+
+import "@testing-library/jest-dom/vitest";
+import { en } from "@snapotter/shared";
+import { cleanup, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it } from "vitest";
+import { RemoveBgControls } from "@/components/tools/remove-bg-settings";
+import { UpscaleControls } from "@/components/tools/upscale-settings";
+
+afterEach(cleanup);
+
+// A user on a CPU-only NUC was surprised AI upscale timed out ("unless my
+// system is just underpowered"). These notices set that expectation up front.
+describe("AI CPU expectation notice (#591)", () => {
+  it("upscale settings warn that it is slow without a GPU", () => {
+    render(<UpscaleControls />);
+    expect(screen.getByTestId("upscale-cpu-note")).toHaveTextContent(
+      en.toolSettings.upscale.cpuNote,
+    );
+    expect(en.toolSettings.upscale.cpuNote).toMatch(/GPU/);
+  });
+
+  it("remove-background settings warn that it is slow without a GPU", () => {
+    render(<RemoveBgControls settings={{}} onChange={() => {}} />);
+    expect(screen.getByTestId("remove-bg-cpu-note")).toHaveTextContent(
+      en.toolSettings["remove-background"].cpuNote,
+    );
+    expect(en.toolSettings["remove-background"].cpuNote).toMatch(/GPU/);
+  });
+});