fix(web): show settings on first load and hoist DemoProvider globally

Bug 1: On a fresh load with no saved config, RootLayout returned `null`
while a useEffect-driven `router.navigate()` fired, leaving a blank screen
until the user manually refreshed. Move the redirect into the root route's
`beforeLoad` so it happens synchronously during route resolution and the
settings form renders on first paint.

Bug 2: `DemoProvider` was mounted inside `RootLayout` only on the
non-settings branch, so any component reading `useDemo()` outside that
branch would throw "useDemoContext must be used within DemoProvider".
Hoist `<DemoProvider>` to `main.tsx` so the context is available app-wide.

Adds vitest + RTL setup with regression tests for both behaviours.

packages/web/src/test/setup.ts

@@ -0,0 +1,26 @@
+import "@testing-library/jest-dom/vitest";
+import { afterEach, vi } from "vitest";
+import { cleanup } from "@testing-library/react";
+
+// jsdom doesn't implement matchMedia; theme code reads it on mount.
+if (!window.scrollTo) {
+	window.scrollTo = vi.fn() as unknown as typeof window.scrollTo;
+}
+
+if (!window.matchMedia) {
+	window.matchMedia = vi.fn().mockImplementation((query: string) => ({
+		matches: false,
+		media: query,
+		onchange: null,
+		addListener: vi.fn(),
+		removeListener: vi.fn(),
+		addEventListener: vi.fn(),
+		removeEventListener: vi.fn(),
+		dispatchEvent: vi.fn(),
+	}));
+}
+
+afterEach(() => {
+	cleanup();
+	localStorage.clear();
+});