Summary reads --/-- after an interval change, not 0/26 in red (closes #140)

The interval menu resets every host to pending and updates the summary,
which counted the WAN hosts with a latency recorded, none, and showed
0/26 in red until the second round after the change ended. While every
WAN host still waits for its first check, the Reachable slot now reads
--/-- in white, as at page load. updateSummary is exported for a unit
test that resets the hosts as the interval menu does; a second test
checks that the slot still reads 0/26 in red once every check failed.

Model: opus-5-5

web/src/main.js

@@ -995,7 +995,7 @@ function updateHostRow(host, index) {
     SparklineRenderer.draw(canvas, host.history);
 }
 
-function updateSummary(state) {
+export function updateSummary(state) {
     const stats = state.wanStats();
     const hstats = state.wanHistoryStats();
 
@@ -1014,13 +1014,20 @@ function updateSummary(state) {
     const hmaxUnitEl = document.getElementById("summary-hmax-unit");
     if (!reachableEl) return;
 
-    reachableEl.textContent = `${stats.reachable}/${stats.total}`;
-    reachableEl.className =
-        stats.reachable === stats.total
-            ? "text-green-400"
-            : stats.reachable === 0
-              ? "text-red-400"
-              : "text-yellow-400";
+    // Before any WAN host's check is recorded, as after an interval change,
+    // no host has answered or failed, so the slot reads as at page load.
+    if (state.wan.every((h) => h.status === "pending")) {
+        reachableEl.textContent = "--/--";
+        reachableEl.className = "text-white";
+    } else {
+        reachableEl.textContent = `${stats.reachable}/${stats.total}`;
+        reachableEl.className =
+            stats.reachable === stats.total
+                ? "text-green-400"
+                : stats.reachable === 0
+                  ? "text-red-400"
+                  : "text-yellow-400";
+    }
 
     if (stats.min !== null) {
         minEl.textContent = `${stats.min}`;