Add testing framework and core tests

[satsdisco]

Summary

Zero tests exist. For a tool that handles private keys, signing, and publishing — this is a gap that needs filling before we add more features.

Scope (start small, grow)

Unit tests (Vitest)

• lib/nostr.ts — event building, relay publishing, npub encoding
• lib/markdown.ts — HTML↔Markdown conversion
• lib/signer.ts — signing interface
• types/nostr.ts — type validation

Integration tests

• Auth flow: NIP-07, NIP-46 connection, nsec import
• Publish flow: article event creation, relay selection, publish result handling
• Draft persistence: save/restore from localStorage

Component tests (React Testing Library)

• Editor: content input, draft auto-save
• PublishModal: relay selection, publish trigger
• ArticleReader: article rendering, comment loading

Setup

• Vitest (already Vite-based, zero config needed)
• React Testing Library for component tests
• Mock nostr-tools relay connections

Acceptance Criteria

• npm test runs and passes
• Coverage on critical paths: signing, publishing, draft persistence
• CI runs tests on PR (GitHub Actions)