Mark where each question starts and give the open question room

[PhilReinking]

The editor now draws a thin line where each question starts, makes the number the anchor under it, and gives the open question 24 px above and below. On phones the side margin grows to 16 px.

Fixes BITS-295.

• The space and line sit on each block's wrapper, not the row, so the margin's rule line still lines up.
• No line on the first block of a list or next to a group's frame.
• The end screen starts with a line too, and gets 24 px above and below when open.
• On phones down to 360 px a group's name stays whole; Add rule and its menu wrap below it together, on the right.
• The open question and end screen cards get a bigger shadow.

Changed from the spec: an open group keeps 8 px. Only an open question gets 24 px, so a group doesn't jump when you click Add rule.

Stacked on #280: no CI here, all checks ran locally.

Tested: Vitest (the new tests fail without the change), lint, vue-tsc, build. By hand at 1440, 1024, 390 and 360 on a 34-question form. Design check passed.

🤖 Generated with Claude Code

[PhilReinking]

OK at b8154f7. No CI (stacked), local runs: Vitest 500, lint, vue-tsc, build. The 5 new tests fail without their change.

Checked at 1440, 1024, 390 and 360: 28 lines, none on a first block, a group or a question after a group. The rule line meets both numbers exactly, the add line sits on the divider, and the open question and end screen get 24 px. At 360 Add rule and the menu wrap together.

[PhilReinking]

Landed in main with #282 (the editor rebuild as one PR); this PR's commits are part of it.