NostrApp rebuilds its QueryClient reactively, but QueryClientProvider reads it only once

`NostrApp` builds its `QueryClient` in a reactive statement
(`src/lib/components/NostrApp.svelte:31`):

```ts
$: queryClient = new QueryClient(mergedQueryClientConfig);
```

but `QueryClientProvider` reads it only at initialization
(`node_modules/@tanstack/svelte-query/dist/QueryClientProvider.svelte`):

```svelte
onMount(() => { client.mount() })
setQueryClientContext(client) // init only
onDestroy(() => { client.unmount() })
```

So if `queryClientConfig`'s identity ever changes — an inline object literal on the
`` tag is enough — a fresh `QueryClient` is constructed and thrown away:

- descendants keep resolving the original client from context;
- the new client is never `mount()`ed;
- on destroy, `unmount()` runs against the *latest* client (never mounted, so its
`mountCount` goes negative), while the client that actually installed the
focus/online listeners is **never unmounted** — one listener leak per `NostrApp`
lifetime.

`NostrApp` also never clears its cache on destroy.

The same init-only behaviour holds in `@tanstack/svelte-query` v6, so this is not
fixed by upgrading.

Determined by code reading of the installed adapter.

## Direction

Create the `QueryClient` once (not in a reactive statement). If reacting to config
changes is wanted at all, it needs a keyed remount of the provider, not a
reassignment.

Start in src/lib/components/NostrApp.svelte:31 and inspect how queryClient is created and destroyed. Confirm the provider behavior described in the issue before making the change. Done means each NostrApp lifetime uses one QueryClient, the client is cleaned up on destroy, and changing config identity does not leak listeners or leave a discarded client.

索引モデルが issue の本文から書いたものです。