pin @nostr-dev-kit/ndk to 2.8.1 instead of workspace:*

Upstream declares the dependency as workspace:*, but the repo has no
pnpm-workspace.yaml and no sibling @nostr-dev-kit/ndk package — so
pnpm install fails with ERR_PNPM_WORKSPACE_PKG_NOT_FOUND on a clean
clone. The shipped pnpm-lock.yaml was resolving to ndk 2.8.1, so pin
to that exact version to match what the lockfile already expects.

Fixes #3.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -39,7 +39,7 @@
     "@fastify/view": "^8.2.0",
     "@inquirer/password": "^1.1.2",
     "@inquirer/prompts": "^1.2.3",
-    "@nostr-dev-kit/ndk": "workspace:*",
+    "@nostr-dev-kit/ndk": "2.8.1",
     "@prisma/client": "^5.4.1",
     "@scure/base": "^1.1.1",
     "@types/yargs": "^17.0.24",