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/README.md

@@ -22,12 +22,31 @@ idf.py -p PORT flash monitor
 
 (`Ctrl-]` exits the monitor.)
 
-The committed [`sdkconfig.defaults`](sdkconfig.defaults) pins an 8MB flash
-size and a custom [`partitions.csv`](partitions.csv) (2MB app partition --
-the default "single app" ~1MB partition runs out of room once the HTTP
-client, TLS, and vendored fonts/QR library are linked in). If your board
-has less flash, you'll need to shrink the app partition and drop features
-to fit.
+The commands above target the dev board this project is built against
+(ESP32-C6-DevKitC-1, 8MB flash) -- the committed
+[`sdkconfig.defaults`](sdkconfig.defaults) pins that flash size and a
+custom [`partitions.csv`](partitions.csv) with dual OTA app partitions (2MB
+each; see [OTA updates](#ota-updates) below), since the default "single
+app" ~1MB partition runs out of room once the HTTP client, TLS, and
+vendored fonts/QR library are linked in.
+
+### Building for the Seeed XIAO ESP32-C6 (production board)
+
+The XIAO has only 4MB of flash, which doesn't fit the dev board's two 2MB
+OTA slots -- it needs its own partition table
+([`partitions_xiao.csv`](partitions_xiao.csv), 1.875MB slots) and flash-size
+setting. Rather than hand-editing `sdkconfig` back and forth between boards,
+use [`build_for_board.sh`](build_for_board.sh), which builds each board into
+its own directory with its own generated config, so switching back and forth
+never clobbers the other:
+
+```
+./build_for_board.sh xiao build
+./build_for_board.sh xiao flash monitor -p PORT
+```
+
+(`./build_for_board.sh devkit ...` does the same for the dev board --
+equivalent to a plain `idf.py`, just consistent with the XIAO invocation.)
 
 ## Configuration (`idf.py menuconfig`)