Add setup detection and content QA

[bnz183]

Summary

• Adds framework auto-detection with confidence scoring and safe copy-only config suggestions.
• Adds read-only content audit for existing posts (frontmatter, duplicate slugs, complex MDX).
• Extends content QA warnings and adds a compact publish checklist before publishing.

Test plan

• pnpm build
• pnpm test
• pnpm test:e2e
• Settings shows setup detection and content audit panels in demo mode
• Publish checklist appears with validation, path, mode, and warnings

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