cardtile editor: every screen answers three questions; paste a YouTube link; coach mark above the card

[cverorg]

Owner's ruler, rule 2: every screen answers what is this / what can I do here / what happens next. Audited seven screens; four had a gap:

• Board: a one-liner under the heading (drag to reorder, tap to change; "Card front" is what visitors see first, other lanes are drawers a tile opens).
• Markdown: a lead above the text ("your whole card as text; Back to the card checks first, changes nothing it can't read").
• Picker: each kind shows its one-line description (was hover-only, invisible on phones) and a line saying where the new tile goes; two-column list.
• Video sheet: labels no longer say "id"; both helper lines rewritten; and the sheet now accepts a pasted YouTube link (youtubeRef: watch?v=, youtu.be, shorts, embed, live, /v/, m./music./nocookie hosts, channel/UC…), bare ids pass through, anything else is left as typed. @handle links are not resolved (needs a lookup).
• Coach mark is a strip above the card at every width; it no longer covers the hero on phones.

Nine locales throughout; w2/three-questions.test.mjs pins titles/descriptions/helper lines per kind, the new lines with their slots, the door text, the YouTube table (+ control) and the no-overlap rule. Verified on the preview Worker (WebKit 390 and 1300). Tests 270/270.