fix(weather): anchor timeline offsets to the Now slot, not wall-clock

Same bug as the propagation map had in 9db9e6a. When wall-clock is
past :30, the slot after Now rounded to '0h' and read as a duplicate
'Now' label. Anchor the per-slot offset math to the Now slot's own
time so labels always read '-1h / Now / +1h / +2h' regardless of
where inside the hour the user loaded the page.

assets/js/weather_map_hook.ts

@@ -734,18 +734,22 @@ export const WeatherMap: WeatherMapHook = {
       const dt = new Date(time)
       const isSelected = time === this.selectedTime
       const isPast = dt.getTime() < now.getTime() - 1800000
-      const offsetH = Math.round((dt.getTime() - now.getTime()) / 3600000)
+      return { time, dt, isSelected, isPast, offsetH: 0, label: "" }
-      return { time, dt, isSelected, isPast, offsetH, label: "" }
     })
 
     const closestIdx = items.reduce((best, item, i) =>
       Math.abs(item.dt.getTime() - now.getTime()) < Math.abs(items[best].dt.getTime() - now.getTime()) ? i : best, 0)
 
+    // Label offsets relative to the "Now" slot, not wall-clock. If we
+    // round from wall-clock, the hour after "Now" rounds to "0h" when
+    // the clock is past the half-hour, which reads as a second "now".
+    const anchorMs = items[closestIdx].dt.getTime()
     items.forEach((t, i) => {
+      const diff = Math.round((t.dt.getTime() - anchorMs) / 3600000)
+      t.offsetH = diff
       if (i === closestIdx) {
         t.label = "Now"
       } else {
-        const diff = t.offsetH
         t.label = diff > 0 ? `+${diff}h` : `${diff}h`
       }
     })