Centralizes browser-mutation origin validation into `src/server/origin/publicOrigin.ts` and wires it through the authz pipeline, replacing the per-route same-origin-only check that 403'd dashboard mutations when served behind a reverse proxy on a different public origin. The new module resolves the allowed public origin from configured base-URL env vars or trusted forwarded headers (only when OMNIROUTE_TRUST_PROXY is set AND the peer is loopback/LAN via peer-stamp), validates Sec-Fetch-Site metadata, and sanitizes Host/Forwarded inputs (rejects control chars, userinfo, path/query in Host).
Reviewed sound; validated locally: authz/public-origin + pipeline suites 27/27 green (incl. invalid-origin reject + configured-origin accept), typecheck clean. Maintainer fix-up: moved the new test from tests/unit/server/ (not collected by any runner — orphan-test gate fail) into tests/unit/authz/. Remaining red CI shards are the pre-existing #4076 Dockerfile heap base-red on `main` (unrelated; de-brittled in the v3.8.40 release line).
Co-authored-by: Thinkscape <Thinkscape@users.noreply.github.com>
