Firmware: OTA client, dual-board build (devkit/XIAO), version reporting, XIAO fixes

- version.txt + esp_app_desc_t version reporting (X-Frame-Version header);
  new ota_update.c checks the server's advertised version against the
  running one and streams+applies an update via esp_https_ota, gated by
  bootloader rollback (marks the image valid only after a full successful
  cycle, so a bad update can't brick a wall-mounted frame).
- Dual-OTA partition tables: partitions.csv (8MB dev board, 2MB slots) and
  new partitions_xiao.csv (4MB XIAO, 1.875MB slots -- the dev board's
  table doesn't fit the XIAO's flash). New build_for_board.sh gives each
  board its own build dir + generated sdkconfig via SDKCONFIG_DEFAULTS
  layering, so switching boards never clobbers the other's config.
- fetch_photo_info()/fetch_face_labels() were using the short
  reachability-check timeout even though the manage-menu path can be the
  first (cold, TLS-handshake-paying) request of a wake cycle -- switched
  to the longer fetch timeout to stop spurious ESP_ERR_HTTP_CONNECT
  failures.
- XIAO: the RF switch that selects onboard vs. external antenna
  (GPIO3/14) isn't initialized by plain ESP-IDF the way Seeed's Arduino
  package does it, leaving WiFi unable to reliably reach the antenna at
  all -- new board_antenna.c powers the switch and selects the onboard
  antenna, gated behind FRAME_XIAO_ANTENNA_INIT (on by default in
  sdkconfig.xiao). Also remaps the EPD DC/RST/BUSY pins, since the dev
  board's defaults (GPIO9/10/11) aren't physically exposed on the XIAO.

firmware/build_for_board.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+# Builds/flashes for a specific board variant. This project targets two:
+#
+#   devkit  ESP32-C6-DevKitC-1 (8MB flash) -- the dev board. This is
+#           also the plain `idf.py` default (sdkconfig/build/), so this
+#           script's devkit mode is mostly for symmetry -- normal
+#           `idf.py build`/`flash` work fine too.
+#   xiao    Seeed XIAO ESP32-C6 (4MB flash) -- the production board.
+#
+# The two need different partition tables (the XIAO's 4MB doesn't fit
+# the dev board's two 2MB OTA app slots -- see partitions_xiao.csv,
+# 1.875MB slots instead) and a different flash-size Kconfig. Rather
+# than hand-editing the shared sdkconfig back and forth (fragile, easy
+# to leave it in the wrong state for whichever board you flash next),
+# each board gets its own build directory and its own generated
+# sdkconfig, seeded from sdkconfig.defaults (shared) with the board's
+# override file layered on top via ESP-IDF's own SDKCONFIG_DEFAULTS
+# mechanism. Switching boards is just switching which one you invoke --
+# neither ever touches the other's config or build output.
+#
+# Usage:
+#   ./build_for_board.sh xiao build
+#   ./build_for_board.sh xiao flash -p /dev/ttyUSB0
+#   ./build_for_board.sh xiao flash monitor -p /dev/ttyUSB0
+#   ./build_for_board.sh devkit build
+#
+# Defaults to "build" if no idf.py subcommand is given.
+
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+cd "$script_dir"
+
+if [ $# -lt 1 ]; then
+    echo "Usage: $0 <devkit|xiao> [idf.py args...]" >&2
+    exit 1
+fi
+board="$1"
+shift
+
+case "$board" in
+    xiao)
+        build_dir="$script_dir/build_xiao"
+        sdkconfig_path="$script_dir/sdkconfig.xiao_local"
+        defaults="$script_dir/sdkconfig.defaults;$script_dir/sdkconfig.xiao"
+        ;;
+    devkit)
+        build_dir="$script_dir/build"
+        sdkconfig_path="$script_dir/sdkconfig"
+        defaults="$script_dir/sdkconfig.defaults"
+        ;;
+    *)
+        echo "Unknown board '$board' -- expected 'devkit' or 'xiao'" >&2
+        exit 1
+        ;;
+esac
+
+args=("$@")
+if [ ${#args[@]} -eq 0 ]; then
+    args=(build)
+fi
+
+# idf.py is normally a shell *function* (defined by ESP-IDF's
+# activate/export script), not a real executable on PATH -- that
+# function isn't inherited by this script's own subshell even if you
+# sourced the activation script first. IDF_PATH (an exported env var,
+# which *is* inherited) lets this work the same way regardless: call
+# the underlying Python module directly.
+if [ -z "${IDF_PATH:-}" ]; then
+    echo "IDF_PATH is not set -- source your ESP-IDF activation/export.sh first" >&2
+    exit 1
+fi
+
+echo "==> Board: $board (build dir: $(basename "$build_dir"), sdkconfig: $(basename "$sdkconfig_path"))"
+exec python "$IDF_PATH/tools/idf.py" -B "$build_dir" -D "SDKCONFIG=$sdkconfig_path" -D "SDKCONFIG_DEFAULTS=$defaults" "${args[@]}"