Previously the row just disappeared whenever battery_estimate_s
couldn't be computed yet, which looked like the feature was gone.
Now it always shows once there's any battery reading at all, with a
placeholder until enough discharge history accumulates (matches the
"Not enough data yet." wording battery_chart.js already uses for the
same situation on the chart).
"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.
Moved out of the Stats tab's side column into a new horizontal bar
shared by every frame page (Photos/Configuration/Stats), sitting
between the page title and the tabs so it's on screen regardless of
which tab is active.
_device_status_bar.html is a new partial included via a device_status
block in app_base.html; device_status_bar.js is the fetch/render/poll
logic extracted from frame_stats.js and adapted to a wrapping row of
label/value pairs instead of a stacked list. frame_stats.html's Device
card and now-single-card .side-col are gone -- Battery history and
Lifetime stats just stack directly.