Add Playwright e2e tests for history panel UI interactions #7

Context

Several PR #4 bugs (stale history on article switch, restore-while-saving race) are frontend-only and can't be caught by unit or integration tests. The history panel has interactive state (toggle, lazy-fetch, preview, restore confirmation) that benefits from browser-level testing.

Proposal

Add lightweight Playwright e2e tests covering:

• Expand history panel → versions load
• Switch articles with panel open → history resets (no stale data)
• Select version → preview renders
• Restore flow → confirmation dialog, article reloads
• Restore blocked during autosave

References

• public/index.html (history panel, fetchHistory, selectVersion, restoreVersion)
• PR feat: version history browser (CE3) #4 review: stale history bug, saving guard bug