feat(web): group questions into screens in the builder

[josue-commits]

Closes #200 (2 of 2).

Stacked on the engine PR (#205). This PR targets feat/screen-groups-engine-renderer. Merge it after that PR, then retarget it to develop.

This is the builder side of screens. Authors join a question to the screen above it, and the screen then shows as one unit everywhere the builder shows the form.

What it adds

• Question list (spine).
  • A chain toggle between two rows joins or splits them. It shows on hover or focus, and it stays visible while the rows are joined.
  • Joined rows share borders, and the first row carries a "Screen N · X questions" chip. Question numbers stay per question.
  • Dragging:
    • within a screen, it reorders;
    • dropping between two members joins the screen;
    • dropping anywhere else leaves it;
    • a question that can't join, dropped inside a screen, lands right after it.
  • Mid-drag, rows move by translation only (no stretching), and the dragged row lifts as its own card. Screen names and chains step aside until the drop.
  • There is no toggle above the first shown question, because nothing can join there.
  • A selected or hovered row draws one outline that follows its shape inside the screen.
• Settings switch. "Show on the same screen as the question above" is always visible, after the Logic card. Below lg the spine is hidden, so this switch is the only way to group there.
  • It can't apply to the first question, schedulers, reveal screens, file uploads, hidden questions, or a screen that already has 10 questions.
  • In those cases it stays focusable and says why.
• Canvas. Selecting a member draws its whole screen as one card. The card is keyed by screen, so switching between members doesn't remount it.
• Preview. The preview walks screen by screen, and "Step N of M" counts screens.
• Logic.
  • Jump targets offer only screen starts.
  • Notes say that a jump runs when the respondent leaves the screen, and warn about an existing rule that targets the middle of a screen or its own screen.
  • The logic canvas draws a frame around each screen, and the map names it.
  • Each select keeps only its own stale value, and new rules default to a target that is actually offered.
• Keeping screens valid. Every edit runs the same normalization: add, delete, reorder, type change, hide or show, layout switch, and restoring a recovered draft. So:
  • no edit leaves a broken screen;
  • the partial point stays after its screen;
  • un-hiding a question into a full screen, or un-hiding a scheduler, reveal or file upload, places it right after the screen.
• One page. The screen controls are hidden, and a note explains that screens only apply to slides. The ids stay in the config, so switching back to slides restores the screens.
• Copy in English and Spanish, including the layout descriptions in the create form and the design panel.

Notes for review

• PM decisions:
  • the switch is always visible, not inside Advanced settings;
  • on a multi-question screen, a message heading keeps the slide title size and doesn't take the initial focus;
  • the solo-type copy uses the gallery's names ("Schedulers" / "agendadores").
• Scheduler panel: the after-booking picker now labels an untitled target "Question N" instead of showing its raw key. This applies to every form.
• One page: drag reordering applies the same join and leave rules, so switching back to slides never finds a broken screen.

Testing

• pnpm typecheck, pnpm lint and pnpm test pass; web has 1122 tests.
• e2e: v19-screens covers the builder: the spine toggle, the Settings switch, the screen canvas, the preview by screens and the jump target list.
  • Chromium, with v18 and v17 in the same run: 24 passed.
  • WebKit: the only failure is the builder preview test that already fails on the base.
  • After the final spine polish, the v19 builder cases were run again in Chromium: 3/3.
• Publish gate passed, no dashes added, DCO and commitlint clean.

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