Replaces the popup's regex-chain humanizeError with a total lookup over
every error code returned by extension/src/service-worker/router/. A
generated test discovers codes via grep so the registry can't drift.
The popup keeps its small set of regex translators for Rust/serde error
phrasing that doesn't go through the router's error vocabulary.
Subsumes B2 — fullscreen consumer lands in the next commit.