Add/csv import options (#420)

* feat(import): add customer and supplier parsing functionality

- Implemented customer file parsing in `lib/import/customers/parser.ts` with support for Excel and CSV formats.
- Created types for detected customer columns and parsed customer rows in `lib/import/customers/types.ts`.
- Added tests for customer classification logic in `lib/import/shared/__tests__/classify.test.ts`.
- Developed classification functions for customers and suppliers in `lib/import/shared/classify.ts`.
- Introduced shared column utility functions in `lib/import/shared/column-utils.ts`.
- Implemented supplier file parsing in `lib/import/suppliers/parser.ts` with validation for various fields.
- Created types for detected supplier columns and parsed supplier rows in `lib/import/suppliers/types.ts`.
- Added tests for supplier column detection and parsing in `lib/import/suppliers/__tests__/column-detector.test.ts` and `lib/import/suppliers/__tests__/parser.test.ts`.

* fix(labels): update 'Svenskt företag' to 'Svenskt företag eller organisation' for clarity

* feat(import): refactor encoding handling for Swedish files and add tests for character preservation

* feat(recapt): implement clearRecaptIdentity function and integrate into logout flow

* feat(bookkeeping): implement copy functionality and next voucher sequence retrieval

* feat(import): enhance customer and supplier import functionality with normalization and event handling

lib/import/shared/column-utils.ts

@@ -0,0 +1,68 @@
+/**
+ * Shared column-detection helpers for register imports
+ * (customers, suppliers, future: articles).
+ */
+
+export function normalize(header: string): string {
+  return header.toLowerCase().trim().replace(/[_\-./]/g, ' ')
+}
+
+export function matchesKeywords(header: string, keywords: string[]): boolean {
+  const normalized = normalize(header)
+  return keywords.some((kw) => normalized === kw || normalized.includes(kw))
+}
+
+/**
+ * Find the first column index whose header matches one of `keywords`,
+ * skipping any indices already taken by other columns.
+ */
+export function findColumn(
+  headers: string[],
+  keywords: string[],
+  taken: Set<number>,
+): number | null {
+  for (let i = 0; i < headers.length; i++) {
+    if (taken.has(i)) continue
+    if (matchesKeywords(headers[i], keywords)) {
+      taken.add(i)
+      return i
+    }
+  }
+  return null
+}
+
+/** Trim a string-or-blank cell, returning null when empty. */
+export function cellOrNull(value: unknown): string | null {
+  if (value === null || value === undefined) return null
+  const str = String(value).trim()
+  return str === '' ? null : str
+}
+
+/** Parse an integer payment term ("30 dagar" → 30) with a default fallback. */
+export function parsePaymentTerms(value: unknown, fallback: number): number {
+  const str = cellOrNull(value)
+  if (!str) return fallback
+  const match = str.match(/-?\d+/)
+  if (!match) return fallback
+  const n = parseInt(match[0], 10)
+  if (isNaN(n) || n < 0 || n > 365) return fallback
+  return n
+}
+
+/**
+ * Normalize an org/personal number to its dedup key (digits only).
+ * Returns null for empty input or strings that contain no digits.
+ */
+export function normalizeOrgNumber(value: string | null): string | null {
+  if (!value) return null
+  return value.replace(/\D/g, '') || null
+}
+
+/**
+ * Normalize an email to its dedup key (trimmed + lowercased).
+ * Returns null for empty/whitespace-only input.
+ */
+export function normalizeEmail(value: string | null): string | null {
+  if (!value) return null
+  return value.trim().toLowerCase() || null
+}