* Studio: mascot images degrade gracefully instead of showing alt text
Mascots loaded from raw public-folder URLs with no error handling, so a
missing or unreachable file (version skew during updates, subpath
mounts, transient blips) painted the alt text, like the empty mascot on
the training start overlay. A shared MascotImg now resolves against
BASE_URL, retries once with a cache-buster, then swaps to a 4 KB
fallback sloth bundled as a data URI that cannot 404. Applied to the
chat greeting, 404 page, training start overlay, auth form, onboarding
splash and wizard.
* Reset mascot retry state via key remount instead of render-time tracking
* Use MascotImg for the artifact generating panel sloth
