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/main.py

@@ -207,10 +207,14 @@ def frame_config(request: Request):
     reachability check. Always returns 200 with current settings
     (defaults if nothing's been saved yet) -- no Immich-configured gate,
     since this doubles as the "is the server up" signal. Also captures
-    the device's running firmware version (X-Frame-Version header) and
-    advertises the uploaded OTA image's version, so the device's update
-    check costs zero extra round trips."""
+    the device's running firmware version and board variant (X-Frame-
+    Version/X-Frame-Board headers -- the latter is how the Gitea
+    auto-update feature learns which release asset to fetch, instead of
+    a user picking it in the web UI) and advertises the uploaded OTA
+    image's version, so the device's update check costs zero extra
+    round trips."""
     reported_version = request.headers.get("X-Frame-Version", "")
+    reported_board = request.headers.get("X-Frame-Board", "")
     with config.locked():
         cfg = config.load()
         cfg.last_seen = time.time()
@@ -221,6 +225,8 @@ def frame_config(request: Request):
             if cfg.device_firmware_version and reported_version != cfg.device_firmware_version:
                 cfg.stats.ota_updates_applied += 1
             cfg.device_firmware_version = reported_version
+        if reported_board:
+            cfg.device_board_variant = reported_board
         config.save(cfg)
     return {
         "refresh_interval_s": _effective_refresh_interval_s(cfg),
@@ -276,7 +282,6 @@ def api_config_save(
     quiet_hours_end: str = Form("07:00"),
     timezone: str = Form("UTC"),
     firmware_update_repo_url: str = Form(""),
-    firmware_board_variant: str = Form("xiao"),
     firmware_auto_update: bool = Form(False),
 ):
     # Immich URL/API key/Gitea token are env-var only (IMMICH_URL/
@@ -309,7 +314,6 @@ def api_config_save(
         if timezone in ALL_TIMEZONES:
             cfg.timezone = timezone
         cfg.firmware_update_repo_url = firmware_update_repo_url.strip()
-        cfg.firmware_board_variant = firmware_board_variant if firmware_board_variant in ("xiao", "devkit") else "xiao"
         cfg.firmware_auto_update = firmware_auto_update
         cfg.stats.config_saves += 1
         config.save(cfg)
@@ -493,13 +497,19 @@ def _fetch_latest_release(cfg: config.FrameConfig) -> dict | None:
 def _apply_gitea_update(cfg: config.FrameConfig) -> str:
     """Downloads the configured Gitea repo's latest release asset for this
     frame's board variant and stages it exactly like a manual
-    POST /api/firmware upload would. Network I/O happens before the lock
-    is taken, matching the load/mutate/save concurrency pattern used
-    elsewhere (see config.locked())."""
+    POST /api/firmware upload would. The board comes from the device
+    itself (device_board_variant, learned from its X-Frame-Board header
+    on GET /frame/config -- see frame_config()), not a user picker, so
+    there's nothing to fetch until a device has checked in at least
+    once. Network I/O happens before the lock is taken, matching the
+    load/mutate/save concurrency pattern used elsewhere (see
+    config.locked())."""
+    if not cfg.device_board_variant:
+        raise HTTPException(400, "No frame has checked in yet -- can't tell which board's build to fetch")
     release = _fetch_latest_release(cfg)
     if not release:
         raise HTTPException(404, "No releases found in the configured Gitea repo")
-    asset_name = gitea_releases.asset_name_for_board(cfg.firmware_board_variant)
+    asset_name = gitea_releases.asset_name_for_board(cfg.device_board_variant)
     asset_url = release["assets"].get(asset_name)
     if not asset_url:
         raise HTTPException(404, f"Latest release has no '{asset_name}' asset")
@@ -524,7 +534,11 @@ def api_firmware_check():
     (gitea_releases.UPDATE_CHECK_INTERVAL_S) -- cheap, since it only reads
     the release's tag name, not its binaries. If firmware_auto_update is
     on and a newer version is found, applies it immediately; otherwise
-    just reports it so the web UI can offer the "Update frame" button."""
+    just reports it so the web UI can offer the "Update frame" button.
+    Applying (auto or manual) needs to know the frame's board, which is
+    learned from the device's own X-Frame-Board header rather than
+    picked by the user -- update_available stays false until a device
+    has checked in at least once, regardless of what Gitea has."""
     cfg = config.load()
     if not cfg.firmware_update_repo_url:
         return {"enabled": False}
@@ -543,9 +557,11 @@ def api_firmware_check():
             config.save(cfg)
         cfg = config.load()
 
-    update_available = bool(
-        cfg.firmware_gitea_latest_version
-    ) and cfg.firmware_gitea_latest_version != cfg.firmware_available_version
+    update_available = (
+        bool(cfg.firmware_gitea_latest_version)
+        and cfg.firmware_gitea_latest_version != cfg.firmware_available_version
+        and bool(cfg.device_board_variant)
+    )
     if update_available and cfg.firmware_auto_update:
         _apply_gitea_update(cfg)
         cfg = config.load()
@@ -553,6 +569,7 @@ def api_firmware_check():
 
     return {
         "enabled": True,
+        "board": cfg.device_board_variant or None,
         "latest_version": cfg.firmware_gitea_latest_version or None,
         "staged_version": cfg.firmware_available_version or None,
         "update_available": update_available,