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/sdkconfig.xiao

@@ -0,0 +1,24 @@
+# Board-specific overrides for the Seeed XIAO ESP32-C6, layered on top
+# of sdkconfig.defaults (which targets the 8MB ESP32-C6-DevKitC-1 dev
+# board) via SDKCONFIG_DEFAULTS -- see build_for_board.sh, which is the
+# supported way to build with this file. Don't set this via a plain
+# `idf.py menuconfig` on the default build; that writes straight into
+# the shared sdkconfig, not this file.
+CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y
+CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions_xiao.csv"
+CONFIG_PARTITION_TABLE_FILENAME="partitions_xiao.csv"
+
+# Powers the XIAO's RF switch and selects its onboard antenna -- without
+# this the softAP/STA radio doesn't reliably reach the antenna at all.
+# See the Kconfig help text (FRAME_XIAO_ANTENNA_INIT) for why.
+CONFIG_FRAME_XIAO_ANTENNA_INIT=y
+
+# EPD wiring: the dev-board defaults (DC=9, RST=10, BUSY=11) aren't
+# physically exposed on the XIAO -- only GPIO 0,1,2,16,17,18,19,20,21,
+# 22,23 are broken out, and 18/19/20 are already the panel's SPI pins
+# (CS/MOSI/CLK) while 0/1/2 are the buttons. Remapped to three of the
+# remaining free pins; see docs/hardware.md for the full XIAO wiring
+# table.
+CONFIG_EPD_PIN_DC=16
+CONFIG_EPD_PIN_RST=17
+CONFIG_EPD_PIN_BUSY=21