fix(editor): render loops in page preview

[DavidBabinec]

Summary

• build Preview page through the authenticated server runtime-preview boundary
• preserve current-draft semantics while resolving loop and media data before publishing
• add abortable loading, error, retry, and toast handling
• add client, component, and server regression coverage plus SITE-016/loop documentation

Root cause

PreviewOverlay called the pure client publisher directly without loopData, so base.loop rendered no items even though the editor canvas and public server renderer prefetched them.

User impact

Preview page now renders loop-backed content consistently with the editor canvas and public page, without publishing the draft.

Verification

• bun test src/__tests__/server/cmsRuntimeHandlers.test.ts src/__tests__/publisher/previewOverlay.test.tsx src/__tests__/persistence/cmsRuntimeClient.test.ts — 44 passed
• bun test — 6,237 passed, 0 failed
• bun run build
• bun run lint
• Chromium same-flow retest at 1440×900: Preview and the live route both rendered ISS-234 LOOP ROW

Fixes #234