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

@@ -32,6 +32,25 @@ const hostState = HOSTS.map(host => ({
   status: 'pending', // 'online', 'offline', 'pending', 'error'
 }))
 
+// 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
+    // Fall back to duration if detailed timing not available (cross-origin)
+    let latency
+    if (entry.requestStart > 0 && entry.responseEnd > 0) {
+      latency = entry.responseEnd - entry.requestStart
+    } else {
+      // For opaque responses, duration includes queuing time but is still more accurate
+      latency = entry.duration
+    }
+    return Math.round(latency)
+  }
+  return null
+}
+
 // Measure latency using HEAD request with Resource Timing API
 async function measureLatency(url) {
   const controller = new AbortController()
@@ -43,6 +62,8 @@ async function measureLatency(url) {
   targetUrl.searchParams.set('_cb', cacheBuster)
   const finalUrl = targetUrl.toString()
 
+  let fetchError = null
+
   try {
     await fetch(finalUrl, {
       method: 'HEAD',
@@ -50,39 +71,32 @@ async function measureLatency(url) {
       cache: 'no-store',
       signal: controller.signal,
     })
-
-    clearTimeout(timeoutId)
-
-    // Get accurate timing from Resource Timing API
-    const entries = performance.getEntriesByName(finalUrl, 'resource')
-    if (entries.length > 0) {
-      const entry = entries[entries.length - 1]
-      // Use responseEnd - requestStart for actual network time
-      // Fall back to duration if detailed timing not available (cross-origin)
-      let latency
-      if (entry.requestStart > 0 && entry.responseEnd > 0) {
-        latency = entry.responseEnd - entry.requestStart
-      } else {
-        // 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 }
-    }
-
-    // Fallback if no timing entry found
-    return { latency: null, error: null }
   } catch (err) {
-    clearTimeout(timeoutId)
-    performance.clearResourceTimings()
+    fetchError = err
+  }
 
-    if (err.name === 'AbortError') {
-      return { latency: null, error: 'timeout' }
-    }
+  clearTimeout(timeoutId)
 
+  // Always check timing - even 4xx/5xx responses have timing data
+  const latency = getTimingForUrl(finalUrl)
+  performance.clearResourceTimings()
+
+  // 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