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>

lib/api/schemas.ts

@@ -2166,6 +2166,10 @@ const ImportedArticleRowSchema = z.object({
   type: ArticleTypeSchema,
   unit: z.string(),
   price_excl_vat: nonNegativeAmount,
+  // ISO 4217 shape only; the execute route validates against the currencies
+  // table and drops unknown codes (mirrors revenue_account). Optional so rows
+  // parsed before this field existed still validate.
+  currency: z.string().regex(/^[A-Z]{3}$/).nullable().optional(),
   vat_rate: vatRatePercent,
   // The execute route re-validates against the chart of accounts (and drops
   // unknown/inactive overrides), so a loose nullable string is enough here.
@@ -2192,6 +2196,9 @@ export const ArticleColumnOverridesSchema = z.object({
   type_col: articleColumnIndex,
   unit_col: articleColumnIndex,
   price_col: articleColumnIndex,
+  // Optional + defaulted so a mapping payload from a client rendered before
+  // this column existed still validates.
+  currency_col: articleColumnIndex.optional().default(null),
   vat_rate_col: articleColumnIndex,
   revenue_account_col: articleColumnIndex,
   cost_price_col: articleColumnIndex,