Swap Proposal H's unified page onto /editor, redirect /display

[WilfordGrimley]

Description

Completes the Proposal H switchover the owner had originally authorized. Nav-redesign PR #313 (2026-07-22) only pointed the "Editor" nav label at /display while the classic grid ProjectEditor stayed live at /editor - matching the design doc's own documented migration plan (§6 step 5: "flip the default nav entry point... keep /editor reachable behind a classic-view link"), but NOT what the owner now says they intended ("all of the changes...should have been to the new editor page (H) and it should be on editor").

This PR does the literal route swap:

• /editor now serves the unified sheet+rail page (DisplayPage.tsx, component unchanged) directly.
• /display is a client-side redirect to /editor, forwarding query params and the URL fragment byte-for-byte (verified - see test plan).
• The classic ProjectEditor component is fully unrouted (kept in-tree, not deleted - a separate later cleanup decision, now documented in its own file header).
• Navbar's "Editor" link points at /editor again.
• pages/print.tsx's empty-state link and internal comments swept for stale /display references; docs/user-guide.md, docs/features/saved-decks.md, and the living proposal-h-display-layout-spec.md updated in place.

Real regression found and fixed as part of the swap: ProjectEditor.tsx's beforeunload unsaved-work guard lived only in that component's function body, never extracted to a shared hook - unrouting it would have silently dropped the guard app-wide (closing/reloading a tab with unsaved cards would warn no one). Ported onto DisplayPage.tsx verbatim.

Known, accepted regression (owner directive: proceed with the swap regardless of issue #272's checklist): ~190 Playwright tests that exercised classic-/editor-only UI (CardSlot grid tiles, SelectedImagesRibbon bulk multi-select, GridSelectorModal-as-standalone-modal, the classic PDF Print! tab, etc. - all confirmed still-open parity gaps per issue #272's own tracking) can no longer reach their subject matter by URL. Each is individually skipped (not deleted, not left silently red) with a standard testInfo.skip(...) citing this swap and issue #272, so CI stays honest and green rather than either lying or drowning in known-red noise.

Unrelated pre-existing bug found while verifying, documented not fixed: useBackendSetter.ts's ?server= URL clean-up (router.replace({ server }, ...)) drops every other query param and the URL fragment on ANY page (reproduced on a direct /editor?server=...&foo=bar#frag load, no redirect involved) - a genuine, separate bug, harmless today since no code builds such a link, documented in docs/troubleshooting.md.

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.
• I have manually tested my changes as follows:
  • npx tsc --noEmit clean.
  • npx jest - 525/525 passing.
  • npx playwright test (full suite, excl. perf) - 324 total: 133 passed, 189 skipped (documented classic-/editor-only coverage, see above), 2 failed (chunkErrorRecovery.spec.ts, confirmed pre-existing/environment flake unrelated to this change - the tested mechanism is app-wide via Layout.tsx, page-agnostic).
  • npx prettier@2.7.1 --check clean on all changed files.
  • python3 .github/scripts/docs_lint.py --strict clean.
  • Manual Playwright-driven screenshots (real MSW-mocked backend, not a live server): desktop /editor populated with a card and the rail open, phone-viewport /editor, and /display?foo=bar#hashval landing on /editor?foo=bar#hashval (params/hash preserved) - confirms the DisplayPage shell renders correctly at the new route on both viewports and the redirect works end-to-end.
• I have updated any relevant documentation or created new documentation where appropriate.
  • docs/user-guide.md, docs/features/saved-decks.md, docs/proposals/proposal-h-display-layout-spec.md (switchover marked DONE), docs/troubleshooting.md (new entry for the unrelated useBackendSetter bug), and in-code comments (pages/editor.tsx, pages/display.tsx, Navbar.tsx, components/ProjectEditor.tsx).

Open items for the owner

1. Issue /display parity inventory: verified remaining gaps vs /editor (switchover checklist) #272's parity checklist is still open (multi-select, FinishSettings placement, cardback gallery chrome, sheet zoom/pan) - this swap proceeds ahead of it per direct instruction, but the checklist itself is unchanged by this PR.
2. The ~190 skipped Playwright tests are real lost coverage until ported to DisplayPage's DOM or formally retired - tracked via their skip-reason citing issue /display parity inventory: verified remaining gaps vs /editor (switchover checklist) #272, not a new tracking item, but worth the owner's awareness at review time.
3. components/ProjectEditor.tsx and its now-unrouted child components are untouched in-tree; deletion is a deliberately separate decision (noted in the file's own header).

[WilfordGrimley]

HOLD (owner ruling 2026-07-23): merges only after #272's parity gaps close — parity is now a high-priority blocker. The branch stays current via rebases as parity PRs land.