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.

server/app/config.py

@@ -91,21 +91,24 @@ class FrameConfig(BaseModel):
     battery_log: list = []
 
     # Device liveness/telemetry: last_seen is touched by every /frame/*
-    # request; device_firmware_version comes from the X-Frame-Version
-    # header the device sends with its config poll.
+    # request; device_firmware_version/device_board_variant come from the
+    # X-Frame-Version/X-Frame-Board headers the device sends with its
+    # config poll (CONFIG_FRAME_BOARD_NAME on the firmware side).
     last_seen: float = 0.0
     device_firmware_version: str = ""
+    device_board_variant: str = ""  # "" until a device has ever checked in
     # Version parsed out of the most recently uploaded OTA image
     # (POST /api/firmware); "" = none uploaded yet.
     firmware_available_version: str = ""
 
     # Gitea-hosted firmware auto-update (see app/gitea_releases.py).
-    # repo_url empty = feature off, no Gitea calls made at all.
-    # board_variant picks which release asset to pull -- must match one of
-    # the names .gitea/workflows/firmware-release-build.yml publishes
+    # repo_url empty = feature off, no Gitea calls made at all. Which
+    # release asset to pull is learned from the device itself
+    # (device_board_variant below, from its X-Frame-Board header) rather
+    # than picked by the user -- must match one of the names
+    # .gitea/workflows/firmware-release-build.yml publishes
     # (firmware-<board_variant>.bin).
     firmware_update_repo_url: str = ""  # e.g. "https://git.example.com/owner/repo"
-    firmware_board_variant: str = "xiao"  # "xiao" or "devkit"
     firmware_auto_update: bool = False  # pull+stage a newer release with no button click
     # Optional Gitea PAT (read-only access is enough) for a private repo's
     # releases; blank is fine for a public repo. GITEA_FIRMWARE_TOKEN env