Read battery once, after the picture is pushed, not at boot

battery_read_percent() was called once at the very start of boot, before
WiFi even connects, and that value was reused both for the manage-menu
overlay and the server report. Taken right after a reset (e.g. the OTA
reboot that immediately precedes it), the rail may still be settling --
plausible source of noisy jumps in reported battery level.

Now there's a single read, in frame_client_run() right before
report_battery(), after the photo (and manage overlay, if shown) is
already on the panel -- the fetch/display work already done this cycle
is the settle time, no delay to guess. The manage overlay no longer
needs an early local reading at all: it shows the server's last-known
value instead, added to the /frame/photo-info response it already
fetches.

server/app/routers/device.py

@@ -365,6 +365,13 @@ def frame_photo_info(frame: Frame = Depends(require_device), db: Session = Depen
         "location_line1": location[0] if location else None,
         "location_line2": location[1] if location and location[1] else None,
         "taken_at": _format_taken_at(exif),
+        # Last value this frame itself reported (see /frame/battery) --
+        # not a fresh reading. Good enough for a glance on the manage
+        # overlay, and lets the device skip a synchronous ADC read (which
+        # would otherwise need to happen before the overlay is composited,
+        # i.e. before the photo it's part of is even pushed to the panel)
+        # just to render this.
+        "battery_percent": frame.battery_percent,
     }