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/skr.classes.ledger.ts

@@ -418,6 +418,7 @@ export class Ledger {
           accountNumber: account.accountNumber,
           debit: Math.abs(balance),
           description: `Closing ${account.accountName}`,
+          postingKey: 40, // Tax-free - internal closing entry
         });
         totalRevenue += Math.abs(balance);
       }
@@ -429,6 +430,7 @@ export class Ledger {
         accountNumber: closingAccountNumber,
         credit: totalRevenue,
         description: 'Revenue closing to P&L',
+        postingKey: 40, // Tax-free - internal closing entry
       });
 
       const revenueClosingEntry = await this.postJournalEntry({
@@ -458,6 +460,7 @@ export class Ledger {
           accountNumber: account.accountNumber,
           credit: Math.abs(balance),
           description: `Closing ${account.accountName}`,
+          postingKey: 40, // Tax-free - internal closing entry
         });
         totalExpense += Math.abs(balance);
       }
@@ -469,6 +472,7 @@ export class Ledger {
         accountNumber: closingAccountNumber,
         debit: totalExpense,
         description: 'Expense closing to P&L',
+        postingKey: 40, // Tax-free - internal closing entry
       });
 
       const expenseClosingEntry = await this.postJournalEntry({