Drop "On battery for", clarify the remaining-estimate label

"On battery for" was clutter next to the actual number people care
about. Relabeled "Est. remaining" to "Est. battery life left" and
dropped the now-unused on_battery_since field from the /queue response.

battery_estimate_s itself is unchanged -- it still needs 2h of span and
a 2% drop within the current discharge cycle (reset on any 5%+ jump,
i.e. a recharge or reflash) before it'll show anything. A frame that's
been power-cycled/reflashed recently won't have an estimate yet; that's
expected, not a regression.

server/app/static/device_status_bar.js

@@ -27,11 +27,8 @@ function renderDeviceStatusBar(device) {
   if (device.battery) {
     rows.push(['Battery', `${device.battery.percent}%`, false]);
   }
-  if (device.on_battery_since) {
-    rows.push(['On battery for', formatDuration(now - device.on_battery_since), false]);
-  }
   if (device.battery_estimate_s !== null && device.battery_estimate_s !== undefined) {
-    rows.push(['Est. remaining', `~${formatDuration(device.battery_estimate_s)}`, false]);
+    rows.push(['Est. battery life left', `~${formatDuration(device.battery_estimate_s)}`, false]);
   }
   for (const [label, value, alert] of rows) {
     const stat = document.createElement('span');
@@ -65,9 +62,8 @@ document.addEventListener('themechange', () => {
   }
 });
 
-// Fast tick: re-renders "Last seen"/"On battery for" from already-
-// fetched data every second so they count up smoothly without hitting
-// the server that often.
+// Fast tick: re-renders "Last seen" from already-fetched data every
+// second so it counts up smoothly without hitting the server that often.
 setInterval(() => {
   if (lastDeviceStatus) {
     renderDeviceStatusBar(lastDeviceStatus);