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/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.