Add Waterfall Chart & Request Inspector

Features Added:
- Interactive waterfall timeline visualization
- Color-coded timing bars (DNS, Connect, SSL, TTFB, Download)
- Request filtering by resource type (HTML, JS, CSS, Images, Fonts)
- Detailed request inspector dialog with timing breakdown
- HAR data extraction from Lighthouse results
- Size/compression metrics display
- Third-party resource identification
- Render-blocking resource detection

Technical Implementation:
- Created lib/har-parser.js for network data extraction
- Built waterfall.html with SVG-based timeline renderer
- Added waterfall.js with interactive controls
- Integrated HAR generation into test runner workflow
- Updated main UI with View Waterfall link

repro_concurrency.js

@@ -0,0 +1,60 @@
+const https = require('https');
+
+const URL = 'https://web-page-performance-test.beyondcloud.technology/api/run-test';
+const TEST_TARGET = 'https://example.com';
+
+function startTest(id) {
+  return new Promise((resolve, reject) => {
+    console.log(`[${id}] Starting test...`);
+    const data = JSON.stringify({
+      url: TEST_TARGET,
+      isMobile: true
+    });
+
+    const req = https.request(URL, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        'Content-Length': data.length,
+        'x-user-uuid': `stress-test-${id}`
+      }
+    }, (res) => {
+      let body = '';
+      res.on('data', chunk => body += chunk);
+      res.on('end', () => {
+        if (res.statusCode === 200) {
+          console.log(`[${id}] Success! Status: ${res.statusCode}`);
+          resolve(true);
+        } else {
+          console.error(`[${id}] Failed! Status: ${res.statusCode}`);
+          // console.error(`[${id}] Body: ${body}`);
+          resolve(false);
+        }
+      });
+    });
+
+    req.on('error', (e) => {
+      console.error(`[${id}] Error: ${e.message}`);
+      resolve(false);
+    });
+
+    req.write(data);
+    req.end();
+  });
+}
+
+async function run() {
+  console.log('Sending 2 concurrent requests...');
+  const results = await Promise.all([
+    startTest('A'),
+    startTest('B')
+  ]);
+  
+  if (results.every(r => r)) {
+    console.log('PASS: Both tests succeeded.');
+  } else {
+    console.log('FAIL: At least one test failed.');
+  }
+}
+
+run();