fix: skip BTRANS/RTRANS in SIE parser to prevent false balance errors (#63)

When Fortnox corrects a voucher, the SIE file contains #BTRANS (added)
and #RTRANS (removed) lines alongside #TRANS (final state). The parser
was summing all three, causing corrected vouchers to appear unbalanced.
Only #TRANS represents the final voucher state — BTRANS/RTRANS are
correction audit trail.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/import/sie-parser.ts

@@ -574,16 +574,24 @@ export function parseSIEFile(content: string): ParsedSIEFile {
         case 'TRANS':
         case 'RTRANS':
         case 'BTRANS': {
-          // #TRANS/#RTRANS/#BTRANS accountNumber {objectList} amount [date] [description] [quantity] [signature]
-          // BTRANS = Added/corrected transaction lines (part of the voucher)
-          // RTRANS = Removed/reversed transaction lines (amounts already have correct sign)
-          // All three must be included for vouchers to balance correctly.
-          // Fortnox/Bokio/Visma only emit #TRANS — this is a no-op for those providers.
+          // #TRANS = final transaction lines (the current state of the voucher)
+          // #RTRANS = removed lines (correction audit trail — original lines that were undone)
+          // #BTRANS = added lines (correction audit trail — new lines that replaced removed ones)
+          //
+          // When a voucher has been corrected, Fortnox/Visma emit all three types.
+          // Only #TRANS represents the final voucher state; #RTRANS and #BTRANS are
+          // supplementary history. We skip RTRANS/BTRANS to avoid double-counting
+          // which would make balanced vouchers appear unbalanced.
           if (!currentVoucher) {
             addIssue(issues, 'error', lineNum, `${tag} outside of VER block`, tag)
             break
           }
 
+          // Skip RTRANS/BTRANS — they are correction audit trail, not final state
+          if (tag === 'RTRANS' || tag === 'BTRANS') {
+            break
+          }
+
           // Parse account and skip object list (in braces)
           let fieldIndex = 1
           const account = parseStringField(fields[fieldIndex++])