the frontend keeps its own copy of the lossless set (settings.js RT_LOSSLESS_FORMATS + the
index.html quality-profile dropdown) — runtime-fetching a yearly-changing list from the backend
isn't worth the coupling. but that duplication IS the exact root cause of #941 (a format added
to one place, not another). so instead of unifying, pin it: two tests parse the frontend lists
and assert they match the backend LOSSLESS_FORMATS. adding a new lossless format now fails CI
until it's added everywhere, instead of silently shipping a half-wired feature.
verified the guard catches drift (not a tautology): a simulated backend-only 'ape' addition
makes the equality fail. 18 lossless tests green, ruff clean.
8aa8fcb94a