Proposal H: scope four /editor parity gaps into unified display spec

[WilfordGrimley]

Description

Docs-only (docs/proposals/, HOLD-style spec work, no code changes): scopes four confirmed feature-parity gaps between /editor and the already-shipped /display page into docs/proposals/proposal-h-unified-display-page.md, matching the doc's existing conventions, and files a corresponding GitHub issue for each.

The owner has decided /display should reach FULL feature parity with /editor before it can supersede /editor's own route. A read-only audit found four /editor capabilities with zero mention anywhere in this doc or in frontend/src/features/display/DisplayPage.tsx:

1. Deck-input landing (§4.1, rewritten this pass) — /display currently mounts zero import surfaces for an empty project, just a link back to /editor. Most urgent given the route-name decision (Open decision 1) — blocks /display from ever starting a project standalone. Issue Proposal H: deck-input landing — /display can't start a project standalone #238.
2. Search Settings (SearchSettings.tsx) — precise/fuzzy search type, DPI/file-size filters, source reordering. Issue Proposal H: Search Settings toolbar parity for /display #239.
3. Project-wide cardback selection (CommonCardback.tsx). Issue Proposal H: Cardback selection toolbar parity for /display #240.
4. Export surfaces beyond PDF (ExportXML.tsx, ExportImages.tsx, ExportDecklist.tsx — ExportPDF.tsx itself already covered via the toolbar's existing Generate PDF/Save-to-Drive mapping). Issue Proposal H: export-beyond-PDF toolbar parity for /display (XML/Images/Decklist) #241.

Also verified a soft-flag: FinishSettings.tsx (cardstock + foil) is a genuinely distinct component from CardQualitySettings (§5's existing PDFGenerator-settings row) — not a naming collision — but has zero mapping anywhere in the doc. Per the task's own scoping, this is handled as a corrective table row + Open decision 7, not a fifth tracked gap/issue.

Changes:

• §4.1 rewritten from "assumed built" to "NOT BUILT" with the actual build spec (mount ImportText/ImportURL/ImportXML/ImportCSV inline when isProjectEmpty).
• §5 component-mapping table gains rows for SearchSettings.tsx, CommonCardback.tsx, the three export components, and the FinishSettings.tsx correction.
• §6 migration/sequencing gains two new small-PR steps (deck-input landing; toolbar instrument parity), renumbering switchover/retire to steps 5/6.
• Open decisions gains items 7 (FinishSettings scoping) and 8 (adding cards to a non-empty /display project, noted not designed).
• Top summary line corrected to acknowledge these as now-known gaps.

Issues filed under milestone "Proposal H: unified display", labeled enhancement: #238, #239, #240, #241.

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate. — N/A, docs-only change, no code touched.
• I have manually tested my changes as follows:
  • npx prettier@2.7.1 --check docs/proposals/proposal-h-unified-display-page.md — clean.
  • python3 .github/scripts/docs_lint.py — clean.
  • Re-read every new paragraph post-Prettier for a real HTML-in-markdown collapse bug Prettier introduced once during editing (inline <Link> text lost surrounding spaces) — caught and fixed, confirmed no other instances via diff-scoped grep.
• I have updated any relevant documentation or created new documentation where appropriate. — this PR is the documentation update; no other docs reference this doc's stale summary.