feat: Add comprehensive testing infrastructure

Implements robust testing setup with Playwright global auth, reusable test
helpers, Docker support, and CI/CD integration with Gitea Actions.

## Changes

### Playwright Setup
- Add global auth setup with storage state reuse (tests/playwright/auth.setup.ts)
- Fix auth setup to clear existing state before fresh login
- Create reusable performOAuthLogin helper (tests/playwright/helpers.ts)
- Configure dotenv loading for environment variables in playwright.config.ts

### Magnitude Configuration
- Update to use Claude Sonnet 4.5 (claude-sonnet-4-5-20250514)
- Create reusable loginFlow helper (tests/magnitude/helpers.ts)
- Fix smoke test to check login page instead of non-existent homepage

### Docker Support
- Add Dockerfile.playwright with non-root user (pwuser) for security
- Uses official Playwright Docker image (mcr.microsoft.com/playwright:v1.49.1-noble)
- Provides consistent testing environment across users and CI/CD

### CI/CD Integration
- Add Gitea Actions workflow (.gitea/workflows/magnitude.yml)
- Runs Magnitude tests on every push and PR
- Starts SurrealDB and Next.js dev server automatically
- Uploads test results as artifacts (30-day retention)

### Documentation
- Add comprehensive testing setup docs to AGENTS.md:
  - Playwright Docker setup instructions
  - CI/CD with Gitea Actions
  - Testing framework separation (Playwright vs Magnitude)
  - Required secrets for CI/CD

### Testing Best Practices
- Separate Playwright (manual + global auth) from Magnitude (automated E2E)
- Reusable helpers reduce code duplication
- Both frameworks work independently

## Testing

- ✅ Playwright auth setup test passes (5.6s)
- ✅ Magnitude smoke test passes
- ✅ OAuth flow works correctly with helper function

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

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

tests/playwright/auth.setup.ts

@@ -1,12 +1,29 @@
-import { test as setup, expect } from '@playwright/test';
+import { test as setup } from '@playwright/test';
+import * as fs from 'fs';
+import * as path from 'path';
+import { performOAuthLogin } from './helpers';
 
 const authFile = 'tests/playwright/.auth/user.json';
 
 setup('authenticate', async ({ page }) => {
-  // For now, just create an empty auth file
-  // TODO: Implement actual OAuth flow when test credentials are available
-  console.log('[Auth Setup] Skipping authentication - implement OAuth flow with test credentials');
+  console.log('[Auth Setup] Starting OAuth authentication flow');
 
-  // Save empty state for now
+  // Clear any existing auth state file to ensure fresh login
+  if (fs.existsSync(authFile)) {
+    fs.unlinkSync(authFile);
+    console.log('[Auth Setup] Cleared existing auth state');
+  }
+
+  // Perform OAuth login using reusable helper
+  await performOAuthLogin(page);
+
+  // Ensure the auth directory exists
+  const authDir = path.dirname(authFile);
+  if (!fs.existsSync(authDir)) {
+    fs.mkdirSync(authDir, { recursive: true });
+  }
+
+  // Save authenticated state
   await page.context().storageState({ path: authFile });
+  console.log(`[Auth Setup] Saved authentication state to ${authFile}`);
 });