fix: never re-bill unchanged listings after a SKU fix (issue #30)

- 'Write all listings' now skips groups whose listing is already written;
  going back to the review step to fix a SKU no longer re-runs (and
  re-bills) analysis for every item
- Moving a photo in or out of a written group resets that group to idle so
  only genuinely changed items get rewritten
- SKU is now editable directly on the listing card (until the item is
  posted), so most SKU fixes don't need the round trip at all — also the
  practical answer to issue #27's want of trimming the letter suffix
- README: document that Vercel's current deploy flow often doesn't prompt
  for env vars — add them in Settings and redeploy (issue #29)