fix: SIE import duplicate check, performance, and UX improvements (#162)

* fix: catch duplicate SIE import early with clear error message

- Add duplicate check in execute route before doing any work (defense in depth)
- Handle duplicate error from execute route in frontend
- Show "Filen har redan importerats" heading instead of generic "Kunde inte läsa filen"

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

* fix: catch duplicate SIE import early and batch account creation for performance

- Add duplicate check in execute route before doing any work (defense in depth)
- Handle duplicate error from execute route in frontend with clear Swedish message
- Replace sequential ensureAccountExists loop (50-100 DB round trips) with single
  batch SELECT + batch INSERT — reduces import time from 3+ min to seconds

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

* fix: update mappings optimistically after creating missing accounts

Previously re-parsed the SIE file after account creation, which could fail
with a 409 duplicate error (leaving the "create accounts" card stuck).
Now optimistically marks created accounts as self-mapped and updates the
preview stats immediately.

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

* fix: address Greptile P2 feedback — error handling and typed errorType prop

- Check batch insert error in executeSIEImport account creation safety net
- Replace brittle string-match error detection with typed errorType prop
- Remove stale file dependency from handleCreateAccounts useCallback

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

---------

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

app/api/import/sie/execute/route.ts

@@ -4,7 +4,7 @@ import { NextResponse } from 'next/server'
 import { requireCompanyId } from '@/lib/company/context'
 import { parseSIEFile, detectEncoding, decodeBuffer } from '@/lib/import/sie-parser'
 import { suggestMappings } from '@/lib/import/account-mapper'
-import { executeSIEImport } from '@/lib/import/sie-import'
+import { executeSIEImport, checkDuplicateImport } from '@/lib/import/sie-import'
 import { BAS_REFERENCE } from '@/lib/bookkeeping/bas-data'
 import { getBASReference } from '@/lib/bookkeeping/bas-reference'
 import type { AccountMapping, SIEAccountMappingRecord } from '@/lib/import/types'
@@ -56,6 +56,15 @@ export async function POST(request: Request) {
     // Parse the SIE file
     const parsed = parseSIEFile(content)
 
+    // Check for duplicate import before doing any work
+    const duplicate = await checkDuplicateImport(supabase, companyId, content)
+    if (duplicate) {
+      return NextResponse.json({
+        error: 'duplicate',
+        message: `Denna fil har redan importerats ${duplicate.imported_at ? new Date(duplicate.imported_at).toLocaleDateString('sv-SE') : ''}`.trim(),
+      }, { status: 409 })
+    }
+
     // Get mappings - either from request or generate new ones
     let mappings: AccountMapping[]