fix: give a clear error when Auth0Provider cannot read the root context

[nandan-bhat]

Summary

Auth0Provider read the router context with a hardcoded useRouteContext({ from: '__root__' }). If an app named or structured its root route differently, TanStack Router threw an opaque invariant error instead of a message that points at the fix. This also documents that login and logout use a full browser navigation on purpose (SDK-10667).

Fix

• Add an optional from prop to Auth0ProviderProps (defaults to __root__), so an app can point the provider at its own root route id.
• Wrap the useRouteContext call so a missing or unknown route id throws a clear SDK error that names the likely cause (auth0BeforeLoad() not set) and the from prop, instead of an opaque router error.
• Document that useLogin and useLogout perform a full browser navigation (not a TanStack Router transition), because the /auth/* routes are handled on the server and the browser must reload to pick up the new session cookie. Added in the JSDoc and the README.

Changes

• src/client/provider.tsx: optional from prop, guarded context read, hard-nav JSDoc.
• README.md: note on the login/logout navigation behavior.
• src/client/provider.test.tsx: tests for the clear error on an unknown route id and for reading context from a custom from id.

Test plan

• npm run build passes
• npm run lint passes
• npm run test passes