resume-link.ts empty-env robustness: align ??->|| so empty-string NEXT_PUBLIC_APP_URL falls to prod host, not a hard URL() crash

resume-link.ts:3 resolves the base URL with '??' (NEXT_PUBLIC_APP_URL ?? prodHost) whereas every other recipient-link resolver uses '||'. '??' only falls through on null/undefined, NOT empty-string, so a literal empty-string NEXT_PUBLIC_APP_URL in Vercel prod would keep '' and hard-crash on new URL('/solicitar-acceso/completar') (no link emitted) instead of falling to the prod host. Fail-hard twin of the venus localhost-leak class (pm-venus VENUS 1386350: empty-but-set env reached a bad fallback). Unreachable today (prod env set-nonempty = https://mars-evolutiva.pensanta.com) = latent robustness only. Fix = 1-line align '??'->'||' in resume-link.ts. Class-S, reversible; fold into next Mars push opportunistically. Surfaced by coder-mars-cc during the pm-venus cross-app localhost-leak sweep 2026-07-20 (Mars recipient links otherwise verified SAFE).