Add battery level reporting (Kconfig-gated) and display orientation

Battery (firmware + server, disabled by default): new battery.c reads
a 2x200k voltage divider via ADC oneshot with curve-fitting calibration
(the ESP32-C6's scheme), maps through a piecewise LiPo discharge curve,
and restores the pin to button duty after each read -- the settled
XIAO ESP32-C6 design shares the back button's GPIO0/A0, time-shared per
wake. Skipped entirely when on mains (a 2x100k VBUS divider into a
spare digital pin -- the 5V pin is dead on battery power, so presence =
mains, where the charging voltage would read misleadingly full) or when
the reading is implausible. The manage overlay gains a battery region
(static outline glyph + "NN%", below the manage QR, all menu levels),
and the device POSTs to the new /frame/battery endpoint after a
successful fetch; the server stores percent + as-of timestamp, exposed
via /api/queue and shown in the web UI. FRAME_BATTERY_ADC_GPIO /
FRAME_VBUS_SENSE_GPIO default to -1 (fully inert on the dev board);
compile-verified both disabled and enabled, hardware bring-up deferred
until the ordered XIAO + batteries arrive.

Orientation (server-side only): new config setting + web UI dropdown
(landscape / portrait / landscape_flipped / portrait_flipped). Photos
are composed/cropped at the logical hanging shape (portrait crops at
480x800, so face-aware crops match how the frame actually hangs), then
rotated losslessly into the panel's native 800x480 byte layout after
dithering -- the device never knows. Face-label anchors are transformed
through the same rotation (logical_to_native()) so they stay attached
to faces on rotated frames. Known documented limitation: the on-device
manage overlay still renders in native orientation, so it appears
sideways on a portrait-hung frame (QRs scan at any rotation; text reads
sideways).

firmware/main/frame_client.h

@@ -37,5 +37,10 @@ esp_err_t frame_wifi_connect_sta(const frame_config_t *cfg);
  * top-right corner linking to the server's config page, held for 30
  * seconds (the device stays awake), then reverted back to the plain
  * photo before proceeding to the normal sleep-interval logic.
+ *
+ * battery_percent (0-100, or -1 for "no reading" -- see
+ * battery_read_percent()) is shown on the management menu overlay and
+ * reported to the server after a successful fetch; -1 skips both.
  */
-void frame_client_run(const frame_config_t *cfg, fetch_action_t action, bool show_management_qr);
+void frame_client_run(const frame_config_t *cfg, fetch_action_t action, bool show_management_qr,
+                       int battery_percent);