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