fix(nip05): replace console.error with debug logger for fetch failures

[cardugarte]

Problem

`src/user/nip05.ts` fires an unconditional `console.error` when a NIP-05 fetch fails:

```typescript
} catch (_e) {
ndk?.cacheAdapter?.saveNip05(fullname, null);
console.error("Failed to fetch NIP05 for", fullname, _e); // cannot be silenced
return null;
}
```

NIP-05 fetch failures are expected behavior — many Nostr users have unreachable or expired NIP-05 domains. Application developers have no way to opt out: the only workaround is monkey-patching `console.error`, which risks hiding real errors.

Closes #394

Fix

Add a `debug` instance scoped to `ndk:nip05` and replace the `console.error`:

```typescript
import debug from "debug";
const d = debug("ndk:nip05");

// in the catch block:
d("Failed to fetch NIP05 for %s: %O", fullname, _e);
```

Silent by default. Opt-in via `DEBUG=ndk:nip05`.

Why this is consistent

Every other NDK module already uses the `debug` package for internal logging (relay connectivity, subscriptions, authentication). The `nip05.ts` module was the only exception — this PR brings it in line with the rest of the codebase.

Changes

• 1 new import (`debug`)
• 1 new module-level constant (`const d = debug("ndk:nip05")`)
• 1 line changed in the catch block

Zero behavior change. The function still returns `null` and saves a negative cache entry on failure.