feat(import): detect and import the article register's Valuta column (#1183)

Fixes #1167. The register export gained a Valuta column in #1166 but
the importer ignored it, so re-imported non-SEK articles silently
became SEK, breaking the export -> edit -> re-import round-trip.

- Column detector recognizes valuta/valutakod/currency (claimed before
  generic columns; no keyword collision with Momskod).
- Parser normalizes to upper-case ISO shape, drops malformed codes
  with a file-level warning, and carries currency per row.
- Execute route validates codes lazily against the currencies table
  (FK stays the backstop when the reference read fails), imports valid
  codes, defaults absent to SEK, and in merge mode only overwrites
  when the file explicitly carries a valid currency.
- Edit step shows a muted currency marker next to non-SEK prices;
  manual column mapping offers Valuta.
- Export docblock caveat removed.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/api/export/articles/route.ts

@@ -11,8 +11,7 @@ import type { Article } from '@/types'
  *
  * Downloads the article register as xlsx (default) or csv. Read-only: viewers
  * may export. Column headers match the article importer's detector keywords so
- * the file round-trips (export → edit → re-import). Exception: the importer
- * does not yet detect Valuta, so re-imported articles default to SEK.
+ * the file round-trips (export → edit → re-import), including Valuta.
  */
 export const GET = withRouteContext(
   'article.export',