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/main/ota_update.c

@@ -0,0 +1,74 @@
+#include <string.h>
+
+#include "esp_app_desc.h"
+#include "esp_crt_bundle.h"
+#include "esp_http_client.h"
+#include "esp_https_ota.h"
+#include "esp_log.h"
+#include "esp_system.h"
+
+#include "ota_update.h"
+
+static const char *TAG = "ota_update";
+
+/* Generous: the image is ~1.2MB and may stream over a WAN reverse
+ * proxy; esp_https_ota resets this per-read, so it's an inactivity
+ * timeout, not a total-transfer cap. */
+#define OTA_HTTP_TIMEOUT_MS 30000
+
+/* Built the same way as every other tools-server URL -- scheme/cert/
+ * token handling all come from build_url()'s conventions. Duplicated
+ * tiny helper rather than exporting frame_client.c's static build_url();
+ * kept byte-identical in behavior (see frame_client.c). */
+static void build_ota_url(char *out, size_t out_size, const frame_config_t *cfg)
+{
+    const char *toolsserver = cfg->toolsserver;
+    size_t len;
+    if (strncmp(toolsserver, "http://", 7) == 0 || strncmp(toolsserver, "https://", 8) == 0) {
+        len = (size_t)snprintf(out, out_size, "%s/frame/firmware", toolsserver);
+    } else {
+        len = (size_t)snprintf(out, out_size, "http://%s/frame/firmware", toolsserver);
+    }
+    if (cfg->access_token[0] != '\0' && len < out_size) {
+        snprintf(out + len, out_size - len, "?token=%s", cfg->access_token);
+    }
+}
+
+void ota_update_if_available(const frame_config_t *cfg, const char *server_version)
+{
+    if (server_version == NULL || server_version[0] == '\0') {
+        return; /* server has no uploaded firmware */
+    }
+
+    const char *running = esp_app_get_description()->version;
+    if (strcmp(server_version, running) == 0) {
+        return; /* already running what the server has */
+    }
+
+    ESP_LOGI(TAG, "Firmware update available: running '%s', server has '%s' -- starting OTA", running,
+             server_version);
+
+    char url[256];
+    build_ota_url(url, sizeof(url), cfg);
+
+    esp_http_client_config_t http_config = {
+        .url = url,
+        .timeout_ms = OTA_HTTP_TIMEOUT_MS,
+        .crt_bundle_attach = esp_crt_bundle_attach,
+        .keep_alive_enable = true,
+    };
+    esp_https_ota_config_t ota_config = {
+        .http_config = &http_config,
+    };
+
+    esp_err_t err = esp_https_ota(&ota_config);
+    if (err != ESP_OK) {
+        /* Not fatal -- the photo already displayed this cycle; we just
+         * try again on a future wake. */
+        ESP_LOGW(TAG, "OTA failed (%s), will retry on a later wake", esp_err_to_name(err));
+        return;
+    }
+
+    ESP_LOGI(TAG, "OTA complete, restarting into '%s'", server_version);
+    esp_restart();
+}