fix: address user feedback — RC preview, bank sync lookback, CSV import robustness (#233)

Three confirmed issues from user feedback:

1. Reverse charge preview now uses per-item VAT rates and correct accounts
   (2645/2647, 2614/2624/2634) instead of hardcoded 25%/2614
2. Bank sync uses 90-day lookback on first sync (when last_synced_at is null)
   instead of hardcoded 7 days for all syncs
3. Bank file import improvements:
   - Shared date normalizer supporting DD.MM.YYYY, DD/MM/YYYY, YYYYMMDD
   - Silent row skips now reported with reason in issues[]
   - Decimal separator mismatch detection in generic CSV
   - Swedish error message with format diagnostics on detection failure
   - Date format selector in column mapping UI

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

app/api/extensions/enable-banking/sync/cron/route.ts

@@ -132,7 +132,13 @@ export async function GET(request: Request) {
       }
 
       const toDate = new Date().toISOString().split('T')[0]
-      const fromDate = new Date(Date.now() - 7 * 24 * 60 * 60 * 1000)
+      // First sync: 90-day lookback (PSD2 max). Subsequent: 7-day window.
+      const isFirstSync = !connection.last_synced_at
+      const lookbackDays = isFirstSync ? 90 : 7
+      if (isFirstSync) {
+        console.log(`[bank-sync-cron] First sync for connection ${connection.id}, using ${lookbackDays}-day lookback`)
+      }
+      const fromDate = new Date(Date.now() - lookbackDays * 24 * 60 * 60 * 1000)
         .toISOString()
         .split('T')[0]