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>

tests/playwright/fixtures.ts

@@ -0,0 +1,26 @@
+import { test as base, Page } from '@playwright/test';
+
+type Fixtures = {
+  authenticatedPage: Page;
+  chatPage: Page;
+  galaxyPage: Page;
+};
+
+export const test = base.extend<Fixtures>({
+  authenticatedPage: async ({ page }, use) => {
+    await page.goto('/');
+    await use(page);
+  },
+
+  chatPage: async ({ page }, use) => {
+    await page.goto('/chat');
+    await use(page);
+  },
+
+  galaxyPage: async ({ page }, use) => {
+    await page.goto('/galaxy');
+    await use(page);
+  },
+});
+
+export { expect } from '@playwright/test';