Add a "Check now" button to the Firmware update card

GET /api/firmware/check's 15-minute throttle meant a genuinely new
Gitea release could sit invisible in the UI for up to that long even
though POST /api/firmware/apply-latest (unthrottled) would've picked
it up immediately. New ?force=true bypasses the throttle for an
explicit check; the button wires it up and surfaces errors instead of
failing silently like the passive poll.

server/app/main.py

@@ -529,7 +529,7 @@ def _apply_gitea_update(cfg: config.FrameConfig) -> str:
 
 
 @app.get("/api/firmware/check", dependencies=[Depends(require_access_token)])
-def api_firmware_check():
+def api_firmware_check(force: bool = False):
     """Throttled check of the configured Gitea repo's latest release
     (gitea_releases.UPDATE_CHECK_INTERVAL_S) -- cheap, since it only reads
     the release's tag name, not its binaries. If firmware_auto_update is
@@ -538,13 +538,18 @@ def api_firmware_check():
     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."""
+    has checked in at least once, regardless of what Gitea has.
+
+    force=true (the "Check now" button) bypasses the throttle and always
+    hits Gitea -- otherwise a genuinely new release can sit invisible in
+    the UI for up to the full throttle interval even though it's already
+    live, since the passive poll won't look again until then."""
     cfg = config.load()
     if not cfg.firmware_update_repo_url:
         return {"enabled": False}
 
     now = time.time()
-    if now - cfg.firmware_update_checked_at >= gitea_releases.UPDATE_CHECK_INTERVAL_S:
+    if force or now - cfg.firmware_update_checked_at >= gitea_releases.UPDATE_CHECK_INTERVAL_S:
         # Deliberately not updated on failure (see below) -- checked_at only
         # advances on a successful reach, so a Gitea outage gets retried
         # every poll instead of waiting out the full throttle interval.