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/routers/api_frames.py

@@ -207,7 +207,6 @@ def api_queue(
             "firmware_available": cfg.firmware_available_version,
             "battery_percent": cfg.battery_percent,
             "battery_as_of": cfg.battery_as_of,
-            "on_battery_since": cfg.battery_history[0][0] if cfg.battery_history else None,
             "battery_estimate_s": battery_estimate_s(cfg),
             "controller_id": cfg.controlled_by_user_id,
             "controller": (
@@ -240,7 +239,6 @@ def api_queue(
                 if snapshot["battery_percent"] >= 0
                 else None
             ),
-            "on_battery_since": snapshot["on_battery_since"],
             "battery_estimate_s": snapshot["battery_estimate_s"],
         },
     }