Always check timing data even on fetch error (4xx/5xx)

Resource Timing API records latency even for error responses.
Now we check for timing data regardless of fetch success/failure,
only reporting unreachable if no timing data is available.

src/main.js

@@ -32,29 +32,9 @@ const hostState = HOSTS.map(host => ({
   status: 'pending', // 'online', 'offline', 'pending', 'error'
 }))
 
-// Measure latency using HEAD request with Resource Timing API
-async function measureLatency(url) {
-  const controller = new AbortController()
-  const timeoutId = setTimeout(() => controller.abort(), REQUEST_TIMEOUT)
-
-  // Add cache-busting parameter
-  const targetUrl = new URL(url)
-  const cacheBuster = `${Date.now()}-${Math.random().toString(36).slice(2)}`
-  targetUrl.searchParams.set('_cb', cacheBuster)
-  const finalUrl = targetUrl.toString()
-
-  try {
-    await fetch(finalUrl, {
-      method: 'HEAD',
-      mode: 'no-cors',
-      cache: 'no-store',
-      signal: controller.signal,
-    })
-
-    clearTimeout(timeoutId)
-
-    // Get accurate timing from Resource Timing API
-    const entries = performance.getEntriesByName(finalUrl, 'resource')
+// Extract timing from Resource Timing API
+function getTimingForUrl(url) {
+  const entries = performance.getEntriesByName(url, 'resource')
   if (entries.length > 0) {
     const entry = entries[entries.length - 1]
     // Use responseEnd - requestStart for actual network time
@@ -66,23 +46,57 @@ async function measureLatency(url) {
       // For opaque responses, duration includes queuing time but is still more accurate
       latency = entry.duration
     }
-      // Clean up the entry
-      performance.clearResourceTimings()
-      return { latency: Math.round(latency), error: null }
+    return Math.round(latency)
+  }
+  return null
+}
+
+// Measure latency using HEAD request with Resource Timing API
+async function measureLatency(url) {
+  const controller = new AbortController()
+  const timeoutId = setTimeout(() => controller.abort(), REQUEST_TIMEOUT)
+
+  // Add cache-busting parameter
+  const targetUrl = new URL(url)
+  const cacheBuster = `${Date.now()}-${Math.random().toString(36).slice(2)}`
+  targetUrl.searchParams.set('_cb', cacheBuster)
+  const finalUrl = targetUrl.toString()
+
+  let fetchError = null
+
+  try {
+    await fetch(finalUrl, {
+      method: 'HEAD',
+      mode: 'no-cors',
+      cache: 'no-store',
+      signal: controller.signal,
+    })
+  } catch (err) {
+    fetchError = err
   }
 
-    // Fallback if no timing entry found
-    return { latency: null, error: null }
-  } catch (err) {
   clearTimeout(timeoutId)
+
+  // Always check timing - even 4xx/5xx responses have timing data
+  const latency = getTimingForUrl(finalUrl)
   performance.clearResourceTimings()
 
-    if (err.name === 'AbortError') {
+  // If we got timing data, return it regardless of fetch error
+  if (latency !== null) {
+    return { latency, error: null }
+  }
+
+  // No timing data - determine error type
+  if (fetchError?.name === 'AbortError') {
     return { latency: null, error: 'timeout' }
   }
 
+  if (fetchError) {
     return { latency: null, error: 'unreachable' }
   }
+
+  // Fetch succeeded but no timing (shouldn't happen)
+  return { latency: null, error: null }
 }
 
 // Get latency color based on value