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/manage_qr_overlay.h

@@ -4,15 +4,15 @@
 
 #include "esp_err.h"
 
-/* 4 fixed corner regions (manage QR, location, date, share QR) plus up
- * to MANAGE_FACE_LABELS_MAX arbitrary-position named-face labels (see
- * manage_face_label_t below). MANAGE_FACE_LABELS_MAX is capped small
- * deliberately, not arbitrarily -- each label is its own malloc'd
- * buffer, and the 4 fixed regions alone already use a meaningful chunk
- * of the ESP32-C6's limited RAM; this keeps worst-case overlay memory
- * well clear of what the WiFi/HTTP stack needs alongside it. */
+/* 5 fixed regions (manage QR, battery indicator, location, date, share
+ * QR) plus up to MANAGE_FACE_LABELS_MAX arbitrary-position named-face
+ * labels (see manage_face_label_t below). MANAGE_FACE_LABELS_MAX is
+ * capped small deliberately, not arbitrarily -- each label is its own
+ * malloc'd buffer, and the fixed regions alone already use a meaningful
+ * chunk of the ESP32-C6's limited RAM; this keeps worst-case overlay
+ * memory well clear of what the WiFi/HTTP stack needs alongside it. */
 #define MANAGE_FACE_LABELS_MAX 4
-#define MANAGE_OVERLAY_MAX_REGIONS (4 + MANAGE_FACE_LABELS_MAX)
+#define MANAGE_OVERLAY_MAX_REGIONS (5 + MANAGE_FACE_LABELS_MAX)
 
 typedef struct {
     uint8_t *buf; /* malloc'd (w/2)*h bytes, packed 2px/byte; owned by the region */
@@ -38,6 +38,7 @@ typedef struct {
     const char *share_url;       /* bottom-left QR + "SCAN TO"/"DOWNLOAD" caption; NULL/empty skips this region */
     const manage_face_label_t *face_labels; /* named-face labels ("level 2" menu); NULL/empty count skips these */
     int face_label_count;                   /* clamped to MANAGE_FACE_LABELS_MAX internally */
+    int battery_percent;                    /* 0-100 shows an icon + percent below the manage QR; -1 skips it */
 } manage_overlay_content_t;
 
 /**