Fix dashboard showing "n/a" for a legitimate 0.0 °C reading

internal/web/assets/app.js

@@ -218,7 +218,7 @@
 
     // Temperature
     const tempEl = document.getElementById('temp-value');
-    if (snap.temperature?.celsius) {
+    if (snap.temperature?.zone) {
       setText('temp-value', snap.temperature.celsius.toFixed(1) + ' °C');
       tempEl.className = 'metric-value ' + levelClass(snap.temperature.celsius, t.temperature_warn_c, t.temperature_crit_c);
     } else {

internal/web/temperature_test.go

@@ -0,0 +1,27 @@
+package web
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestAppJS_TemperatureNAUsesZoneNotCelsius guards against regressing issue
+// #64: a legitimate 0.0 °C reading must not render as "n/a". A failed
+// collection is distinguished by an empty zone (Temperature.Zone is always
+// set on a successful reading), not by the celsius value's truthiness —
+// 0 is a valid, falsy reading that a `snap.temperature?.celsius` check would
+// wrongly treat as missing.
+func TestAppJS_TemperatureNAUsesZoneNotCelsius(t *testing.T) {
+	data, err := assetsFS.ReadFile("assets/app.js")
+	if err != nil {
+		t.Fatalf("read app.js: %v", err)
+	}
+	src := string(data)
+
+	if strings.Contains(src, "snap.temperature?.celsius)") || strings.Contains(src, "snap.temperature && snap.temperature.celsius)") {
+		t.Errorf("app.js must not gate the temperature display on celsius truthiness (0 °C is a valid reading)")
+	}
+	if !strings.Contains(src, "snap.temperature?.zone") {
+		t.Errorf("expected app.js to key the temperature \"n/a\" fallback off snap.temperature?.zone")
+	}
+}