TSA-267: undo verification ctrl z

[MatiStb]

Closes #267

What

Adds the frontend half of #56 — Ctrl+Z undoes the last page verification and returns the user back to that page, transcribed again, with its original transcription.

• Ctrl+Z / Cmd+Z in viewing mode after confirming a page reverts it and moves the cursor back to it
• The optimistic state is rolled back on failure — not just the page status, but the cursor and the full snapshot saved in the slice's rollback map ([FE] Roll back the full optimistic state, not just page status #144)
• The shortcut is ignored while a textarea/input has focus, so Ctrl+Z in the edit box undoes your typing instead ([FE] Undo the last verification with Ctrl+Z #267 AC)
• The undone page can be confirmed again ([BE] Allow a page to be confirmed again after undo #147)
• Ctrl+Z added to the shortcuts list on the verification header; it was already in the key map in docs/06-verification-ui.md

Why

The backend endpoint POST /pages/:id/undo does not exist yet (#147). This PR ships a dev-only mock of it (guarded by process.env["NODE_ENV"] !== "production") so the flow can be exercised locally until #147 lands. It is therefore not available on the deployed QA environment.

What changed

• FE — apps/frontend/src/pages/verification/verification.tsx: handleUndo (guards on lastVerifiedPageId, isVerifying, isEditing; rolls back on rejection with a notification)
• FE — use-verification-keyboard.hook.ts: keydown listener for Ctrl/Cmd+Z (skips input/textarea/select/contentEditable, event.repeat)
• FE — pages.slice.ts / actions.ts: undoPage thunk + undoOptimistic, lastVerifiedPageId, selectLastVerifiedPageId; verification-header.tsx hints Ctrl+Z Undo
• shared — UndoPageResponseDto ({ pageId, status, transcription })
• BE mock (dev only) — POST /api/v1/pages/:id/undo in page.controller.ts / page.service.ts, reusing DocumentGetPagesTranscriptionResponseDto

How to test

Local environment only — the mock endpoint is disabled in production.

1. Start backend and frontend locally (npm run dev -w apps/backend and npm run dev -w apps/frontend).
2. Create an account for the user owning the test document. The account used during development, undo-mock@test.dev / Password123! (user id 7), only exists in the local database where it was created — it is not part of the seed, so it cannot be reused by someone else's machine. Either sign up your own user or insert rows locally.
3. Have a document with at least one transcribed page for that user (through the normal upload pipeline, or seeded locally).
4. Open the verification screen for the document and confirm a page (Enter or the check button). The page becomes confirmed and the cursor advances.
5. Press Ctrl+Z → you are put back on that page, it shows transcribed again with the original transcription.
6. Confirm the same page again → it can be confirmed after undo ([BE] Allow a page to be confirmed again after undo #147).
7. Inside the edit textarea, press Ctrl+Z while typing → it undoes your typing; the page is untouched.
8. Failed undo (e.g. backend stopped, or nothing to undo) → the screen stays exactly as it was and a message shows: "The undo could not be completed. The page state has been restored."

Verified

• Confirming a page and pressing Ctrl+Z puts the user back on that page, transcribed again
• The undone page shows its original transcription
• A failed undo leaves the screen as it was, with a message
• Ctrl+Z inside the edit textarea undoes typing, not the verification
• The page can be confirmed again after undo
• POST /api/v1/pages/:id/undo → 200 with { pageId, status: "transcribed", transcription }; DB reverts (status → transcribed, verified_by → null)

[arturobdev]

I noticed the confirm → undo → confirm AC cannot really be validated with the current dev mock.
The UI advances after the second confirm, but the DB still keeps the page as transcribed, which matches the known backend issue from #147.

Since #147 is still pending, should this AC remain blocked by that ticket instead of being tested against the temporary mock?

[IlliaMatushchak]

After resolving conflicts and merge 'PageStatus' dissapear from import in page.service.ts and causes the error. I don`t now why. But if it happens, just check and fix it.

[arturobdev]

I tested it locally and it works correctly.

correct → undo → correct works now, and the event history is preserved using attempt:

contextWords still comes back as an empty array from the undo endpoint, but that behavior comes from the backend implementation merged in #343, so there is nothing to change on the frontend side here.