Whiteboard and tasks-source save/clear used to just tell the user to
reload the page to see the change. Both endpoints always assign a
successful "set" to the calling user, so the new state is fully known
client-side already -- rewrite the "Currently using/showing ..." block
in place instead, no server round trip or reload needed.
Also adds .gitea/workflows/firmware-build-check.yml: builds both board
variants (devkit, xiao) on every push touching firmware/**, unlike
firmware-release-build.yml which only builds on a version.txt bump.
Verified both builds succeed locally against the actual ESP-IDF
toolchain before wiring this in.
