~anthropic/claude-sonnet-latest → B (6:4)

Side B fixes a real, user-facing bug (hardcoded staging hostnames breaking OAuth redirects on custom hosts) by consolidating URL logic into a shared util and using env/window.location, plus fixes CI deploy targeting—concrete, deployable value across multiple files. Side A is a refactor (Deque->Vec) that changes storage semantics and adds a test, which is reasonable cleanup but lower-impact than fixing broken redirect/deploy behavior in production.