feat(editor_v2): edit API page details as fields, not YAML

[phrogwrld]

Stacked on #2991 (version history). Review that first; once it merges, this PR's base moves to v2-rewrite.

Summary

A content API page keeps its details (title, introduction, description, path, category, visibility and form) in api_v2's own columns. Until now the editor round-tripped them through YAML frontmatter. With this PR, the editor edits them as fields and converts only the Markdown body.

• Fields: src/workspace/page-details.ts (the PageDetails type, detailsOf, sameDetails, detailsProblems) and page-details-panel.tsx. The details autosave beside the body, and each field keeps its own undo.
• Body-only codec: an API page's Markdown view shows only its body. A body that hasn't changed is saved exactly as the API sent it.
• Browser drafts are unchanged: they keep their YAML frontmatter.
• History: the version preview renders the version's title and introduction above its body. Restore takes the version's own fields.
• Download my version: now saves the whole unsaved page as page.json, in the shape a save sends: { ...details, body_markdown }. Before, it saved only the body, which would have lost unsaved details.

Test plan

• apps/editor_v2: format, lint, typecheck, boundaries, the full suite (1068), and build with VITE_API_ORIGIN set
• Browser, on a content API page:
  • edit the title, introduction and details;
  • Save or Publish;
  • check that History previews and restores a version's details;
  • check that "Download my version" includes the details;
  • check that a browser draft still edits its YAML.

[github-actions]

Amplify PR previews

App	Status	URL
forms	⏭ Not affected	—
landing	⏭ Not affected	—
chat	⏭ Not affected	—
form-builder	⏭ Not affected	—
analytics	⏭ Not affected	—

Commit: 22c8311 · Branches auto-delete when this PR closes.

ℹ️ feature-flagging uses a single shared preview slot at a fixed URL (preview.feature-flagging.sandbox.alpha.gov.bb) — GitHub login works, but only one FF preview exists at a time (the most recent FF PR build wins). It reads/writes sandbox flag data.