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/README.md

@@ -49,6 +49,8 @@ Under **ESPresso Frame Configuration**:
 | `FRAME_COMBO_BUTTON_GPIO` | 1 | Menu/reset button GPIO (-1 to disable). Must be 0-7 |
 | `FRAME_COMBO_SOFT_RESET_HOLD_MS` | 3000 | How long the combo button must be held (then released) to soft-reset |
 | `FRAME_COMBO_FACTORY_RESET_HOLD_MS` | 15000 | How long the combo button must be held to factory-reset |
+| `FRAME_BATTERY_ADC_GPIO` | -1 (disabled) | Battery voltage-divider ADC GPIO; see the Battery section below |
+| `FRAME_VBUS_SENSE_GPIO` | -1 (disabled) | USB-power sense GPIO for hiding the battery indicator on mains |
 
 Under **E-Paper Display (epd7in3e) Configuration**: SPI/GPIO pin
 assignments and SPI clock speed -- see
@@ -177,6 +179,37 @@ If there's nothing to go back to yet (freshly provisioned, or you've
 already gone back as far as there is history), it's a no-op -- the
 current photo stays exactly as it was, no flash on the panel.
 
+## Battery (XIAO ESP32-C6)
+
+Disabled by default (`FRAME_BATTERY_ADC_GPIO = -1`) -- correct for a
+dev board with no battery wired. On the intended production board (Seeed
+XIAO ESP32-C6) with a 1S LiPo:
+
+**Wiring** (beyond soldering the battery itself to the BAT+/BAT- pads on
+the XIAO's underside -- note the polarity markings there, and that
+JST-PH battery connector polarity is *not* standardized, so verify with
+a multimeter before first plug-in):
+
+- **Battery sense**: a 2x200k voltage divider from BAT+ to GND, midpoint
+  into an ADC-capable pin (GPIO 0-6; the settled design shares the back
+  button's GPIO0/A0 -- the high-impedance divider coexists fine with the
+  button, and firmware time-shares the pin with a brief ADC read once
+  per wake). Set `FRAME_BATTERY_ADC_GPIO` to match.
+- **Mains detection** (optional): a 2x100k divider from the 5V pin
+  (which only carries voltage when USB is plugged in) into any spare
+  GPIO -- plain digital high/low, no ADC needed. The divider is
+  required: raw 5V exceeds the 3.3V pin limit. Set
+  `FRAME_VBUS_SENSE_GPIO`.
+
+**Behavior**: once per wake the firmware reads the battery voltage,
+converts it to a percent via a LiPo discharge curve, shows it (battery
+icon + "NN%") below the "scan to manage" QR box whenever the management
+menu is up, and reports it to the server (`POST /frame/battery`), which
+displays it in the web UI with an "as of" timestamp. All of that is
+skipped when on mains power (the charging voltage would read
+misleadingly full), when the reading is implausible (no battery
+attached), or when the feature is disabled.
+
 ## Managing the queue, soft-resetting, and factory-resetting
 
 One more button, wired between GPIO1 and GND (same wiring style as the