feat: add opening balance import functionality (#238)

- Implemented OpeningBalanceResultStep component to display results of the import process, including success messages and error handling.
- Created OpeningBalanceUploadStep component for file upload with drag-and-drop support, including validation for accepted file types.
- Developed column detection logic in column-detector.ts to identify account number, name, debit, credit, and balance columns based on headers and data.
- Added parser functionality in parser.ts to handle parsing of opening balance files, including validation and BAS account matching.
- Created tests for column detection and parsing logic to ensure accuracy and reliability.
- Defined types for detected columns and parsed rows in types.ts to improve type safety and clarity in the codebase.

lib/api/schemas.ts

@@ -599,3 +599,16 @@ export const SaveGapExplanationSchema = z.object({
   gap_end: z.number().int().positive(),
   explanation: z.string().min(1).max(500),
 })
+
+// ============================================================
+// Opening balance import schemas
+// ============================================================
+
+export const OpeningBalanceExecuteSchema = z.object({
+  fiscal_period_id: uuid,
+  lines: z.array(z.object({
+    account_number: accountNumber,
+    debit_amount: nonNegativeAmount,
+    credit_amount: nonNegativeAmount,
+  })).min(2, 'At least two lines are required for double-entry'),
+})