Add per-field copy buttons so drafts match the target site's form

Depop and Vinted's listing forms both have separate fields (price,
brand, size, condition) - the single "copy everything" block meant
manually splitting that text apart to fill out the other site's form
field by field. Each field now has its own compact copy button, so
"copy price -> paste into price field -> copy brand -> paste into
brand field" is a direct one-to-one motion instead. A "Copy all"
button stays at the bottom for anyone who still wants the whole block.

Empty fields (no brand tagged, etc.) show "-" with a disabled copy
button rather than an enabled button that copies nothing.

This does not add any automation - it's still purely about making the
manual copy/paste into Depop's or Vinted's own form faster, which is
what was actually asked for after ruling out auto-filling the form
itself (blocked by file-upload restrictions, no category mapping
between the two sites' taxonomies, and being automation of a public
tool at scale - all discussed and rejected earlier).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>