Hooks throw RxNostrAlreadyDisposedError after NostrApp is destroyed

`NostrApp` disposes its `rxNostr` on destroy
(`src/lib/components/NostrApp.svelte:38-40`) but never clears the module-level
`app` store, so the disposed instance stays published.

`useReq()`'s very first statement reads `rxNostr.getDefaultRelays()`
(`src/lib/stores/useReq.ts:25`), which throws `RxNostrAlreadyDisposedError` on a
disposed instance. Verified on `main` (609f8ed).

Any hook invocation after the provider is torn down is therefore an uncaught throw
rather than a graceful degradation. Reachable via a route change that leaves a
child mounted for a tick, an `{#if}` toggle around ``, or SSR where
`onDestroy` runs at the end of render.

## Direction

Clear the published instance on destroy, and have the hooks treat "no usable
rxNostr" as a state rather than letting the throw escape.