convey/import: adopt wave 0 error primitives for detail view (wave 1)
Migrate the detail bootstrap fetch to apiJson(...). Split failure UI so #importMeta shows a prominent .surface-state-refresh-error banner with err.serverMessage while the overview, import JSON, and imported JSON panels fall back to neutral unavailable copy. The literal Pending string remains reserved for legitimate server-side pending state.
Co-authored-by: OpenAI Codex <codex@openai.com>