Add battery widget (device's own last-reported level, no live upstream)

Shows Frame.battery_percent/battery_as_of, already set by every device
wake-on-battery report, plus routers/common.py's existing
battery_estimate_s time-remaining estimate -- nothing new to fetch or
cache. Compact (icon + percent) or detailed (+ estimate, last report
age) display mode. No button actions.

server/app/grid.py

@@ -27,6 +27,13 @@ GRID_SHORT = 5
 # without truncating on every row; weather needs enough room for its
 # hourly/daily strips to stay legible (its current/multi_city modes
 # would tolerate smaller, but every mode shares one footprint value).
+# battery is just an icon + a percent (+ two optional small lines in
+# "detailed" mode) -- legible even at a single cell, like photos/static.
+# NOTE: a 1x1 widget-box on a narrow mobile canvas can clip its own
+# gear/remove buttons behind theme.css's overflow: hidden (their fixed
+# pixel offsets overflow the box's clipped width) -- a pre-existing
+# layout gap that already affects photos/static at 1x1 too, not fixed
+# here; see the finding called out where this was discovered.
 MIN_FOOTPRINT: dict[str, tuple[int, int]] = {
     "photos": (1, 1),
     "calendar": (3, 2),
@@ -35,6 +42,7 @@ MIN_FOOTPRINT: dict[str, tuple[int, int]] = {
     "static": (1, 1),
     "text": (2, 1),
     "weather": (2, 2),
+    "battery": (1, 1),
 }
 
 Rect = tuple[int, int, int, int]  # (x, y, w, h)