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.
