Harden DeckHub Preflight exports and validation

[Hol-ick]

What changed

• generalize the Preflight concept-card profile
• split large Markdown exports into filtered clipboard copy and full-file download
• add accessible filter labels and inline export feedback
• add six validator regression tests and run them in CI
• update Next.js to 16.2.10 and pin the Turbopack root

Why

The last Preflight polish commit was still outside main, full-deck clipboard exports could become unreliable for large review sets, and the detector rules had only manual regression coverage. The local workspace also produced an incorrect-root warning because another lockfile exists under the user home.

Impact

Maintainers can review large Anki TXT decks with safer exports and clearer feedback. CI now protects the false-positive cases refined during the previous pass. The Next.js patch removes the known high-severity audit findings.

Validation

• npm run frontend:test — 6 passed
• npm run frontend:lint
• GITHUB_PAGES=true npm run frontend:build
• npm run catalog:check
• npm run backend:test — 11 passed
• npm --prefix frontend audit --audit-level=high
• browser smoke test for analysis, filtered copy, review queue, download state, and 390px overflow