feat: Add Playwright testing scaffolding infrastructure

Implements complete Playwright testing infrastructure for manual MCP testing and
as foundation for Magnitude tests.

Created:
- playwright.config.ts - Main configuration with setup project
- tests/playwright/auth.setup.ts - Authentication setup (placeholder)
- tests/playwright/fixtures.ts - Custom fixtures for authenticated/page contexts
- tests/playwright/helpers/chat.ts - Chat interaction helpers
- tests/playwright/helpers/galaxy.ts - Galaxy visualization helpers
- tests/playwright/helpers/node.ts - Node creation/management helpers
- tests/playwright/smoke.spec.ts - Basic smoke tests
- .env.test - Environment variables template

Updated:
- package.json - Added test:playwright scripts
- .gitignore - Added tests/playwright/.auth/ exclusion

Ready for manual Playwright MCP testing and Magnitude test creation.

Resolves plan: 01-playwright-scaffolding.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -42,6 +42,7 @@ next-env.d.ts
 /playwright-report/
 /playwright/.cache/
 .playwright-mcp/
+tests/playwright/.auth/
 
 # claude settings (keep .claude/CLAUDE.md but ignore user settings)
 .claude/settings.local.json