fix: align curate-html-direct error shape with daemon's `kind` discriminator

The daemon emits HtmlWriteError objects keyed on `kind` (see
src/server/infra/render/writer/html-writer.ts), but the web UI parser
expected `code`. The type guard filtered out every error on a
validation-failed payload, so the error list rendered as empty.

Renames CurateHtmlWriteError.code → kind, updates isWriteError to
check obj.kind, and updates the section renderer + React key to read
error.kind. Adds a unit test that round-trips a realistic path-exists
payload through parseCurateHtmlDirectResult to lock the contract.