Consolidate firmware UI; server learns board from the device, not a picker

All firmware-related controls (manual upload, Gitea repo URL,
auto-update checkbox, detected board, Update frame button) now live in
one "Firmware update" card instead of being split across the main
Settings form and a separate card.

The board variant used to pick a Gitea release asset was a dropdown
the user had to set by hand and could get wrong. The device now
reports it itself via a new X-Frame-Board header (CONFIG_FRAME_BOARD_NAME,
"devkit" by default, "xiao" in sdkconfig.xiao) on every /frame/config
poll, stored as device_board_variant -- the server learns it instead.
Update checks/applies are gated on the board being known, since there's
nothing to fetch until a device has checked in at least once.

firmware/main/Kconfig.projbuild

@@ -1,5 +1,20 @@
 menu "ESPresso Frame Configuration"
 
+    config FRAME_BOARD_NAME
+        string "Board variant name, reported to the server"
+        default "devkit"
+        help
+            Sent as the X-Frame-Board request header on every
+            GET /frame/config poll, so the server can learn which board
+            this device is and automatically fetch the right OTA build
+            from a configured Gitea repo's releases -- no manual "which
+            board" picker in the web UI. Must match one of the asset
+            names .gitea/workflows/firmware-release-build.yml publishes
+            (firmware-<name>.bin): "devkit" (this default, for the
+            plain ESP32-C6-DevKitC-1 build) or "xiao" (set via
+            sdkconfig.xiao for the Seeed XIAO ESP32-C6 build -- see
+            build_for_board.sh).
+
     config FRAME_XIAO_ANTENNA_INIT
         bool "Select onboard antenna on Seeed XIAO ESP32-C6 (RF switch init)"
         default n

firmware/main/frame_client.c

@@ -342,9 +342,13 @@ static bool json_extract_string(const char *json, const char *key, char *out, si
 /* GETs the server's /frame/config -- doubles as both the reachability
  * check (any completed HTTP response means the socket-level connection
  * succeeded), the source of the server-configurable refresh interval,
- * and (via the X-Frame-Version request header / firmware_version
- * response field) the device's OTA update check -- piggybacked on a
- * request already made every wake, no extra round trip. */
+ * and (via the X-Frame-Version/X-Frame-Board request headers and
+ * firmware_version response field) the device's OTA update check --
+ * piggybacked on a request already made every wake, no extra round
+ * trip. X-Frame-Board lets the server learn which board this device is
+ * (CONFIG_FRAME_BOARD_NAME) so it can pick the right Gitea release
+ * asset itself, instead of a user manually selecting a board in the
+ * web UI. */
 static frame_server_config_t fetch_frame_config(const frame_config_t *cfg)
 {
     frame_server_config_t result = {
@@ -364,6 +368,7 @@ static frame_server_config_t fetch_frame_config(const frame_config_t *cfg)
     };
     esp_http_client_handle_t client = esp_http_client_init(&config);
     esp_http_client_set_header(client, "X-Frame-Version", esp_app_get_description()->version);
+    esp_http_client_set_header(client, "X-Frame-Board", CONFIG_FRAME_BOARD_NAME);
 
     esp_err_t err = esp_http_client_open(client, 0);
     if (err != ESP_OK) {

firmware/sdkconfig.xiao

@@ -8,6 +8,8 @@ CONFIG_ESPTOOLPY_FLASHSIZE_4MB=y
 CONFIG_PARTITION_TABLE_CUSTOM_FILENAME="partitions_xiao.csv"
 CONFIG_PARTITION_TABLE_FILENAME="partitions_xiao.csv"
 
+CONFIG_FRAME_BOARD_NAME="xiao"
+
 # 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.