feat: Enhance journal entry and transaction handling with posting keys

- Added posting key support to SKR03 and SKR04 journal entries and transactions to ensure DATEV compliance.
- Implemented validation for posting keys in journal entries, ensuring all lines have a posting key and that they are consistent across the entry.
- Introduced automatic account checks to prevent posting to accounts that cannot be directly posted to (e.g., 1400, 1600).
- Updated account validation to include checks for debtor and creditor ranges.
- Enhanced invoice booking logic to include appropriate posting keys based on VAT rates and scenarios.
- Created a new module for posting key definitions and validation rules, including functions for validating posting keys and suggesting appropriate keys based on transaction parameters.
- Updated tests to cover new posting key functionality and ensure compliance with accounting rules.

ts/skr03.data.ts

@@ -159,6 +159,7 @@ export const SKR03_ACCOUNTS: IAccountData[] = [
     accountType: 'asset',
     skrType: 'SKR03',
     description: 'Trade receivables',
+    isAutomaticAccount: true, // Automatikkonto - cannot be posted to directly, use debtor accounts (10000-69999)
   },
   {
     accountNumber: '1500',
@@ -199,6 +200,7 @@ export const SKR03_ACCOUNTS: IAccountData[] = [
     accountType: 'liability',
     skrType: 'SKR03',
     description: 'Trade payables',
+    isAutomaticAccount: true, // Automatikkonto - cannot be posted to directly, use creditor accounts (70000-99999)
   },
   {
     accountNumber: '1700',