feat(pay): explain wallet errors instead of leaking protocol codes

NOT_REGISTERED is the first wall every new pool user hits: the viewing key
is published on-chain once, and until then the pool refuses deposits. The
widget now says exactly that, and what to do about it (shield once in your
wallet, wait ~10 blocks, come back), plus plain-language cases for
screening rejections, insufficient balance, and dismissed prompts. Unknown
errors still pass through verbatim. 27 tests green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>