[HTExplicit]

Problem

The admin API-key account form passed a host-only endpoint such as api2.aigcbest.top to the upstream model-sync preview. The preview rejected it as an invalid URL before account creation, while other paths handled endpoint input differently.

Fix

• Normalize host/path shorthand to https://... in the account form and in the backend credential/preview path.
• Normalize nested adaptive protocol endpoints as well.
• Keep explicit schemes unchanged so the existing URL allowlist and private-host checks remain authoritative.
• Add regression coverage for normalization and API-key model-sync preview with a host-only endpoint.

Validation

• go build ./...
• Go service and admin handler targeted tests
• Frontend account modal, credential-builder, and model-capacity tests (117 account/credential tests plus 31 capacity tests)
• ESLint, vue-tsc, gofmt, and git diff --check