feat(fusion_accounting_reports): XLSX export wizard

Adds a TransientModel wizard fusion.xlsx.export.wizard that lets users
pick a report type, date range, and comparison mode, then runs the
engine and produces an XLSX via xlsxwriter (in-memory).

The wizard exposes a download field that becomes available after export
finishes. Works on P&L, Balance Sheet, Trial Balance, and General Ledger.
Comparison columns are written when the engine returns a comparison_period
in the result.

Also wires the controller's /fusion/reports/export_xlsx endpoint to drive
the wizard and return base64-encoded XLSX bytes (replaces the not_implemented
placeholder).

Tests: 2 new (test_xlsx_export.py) + 1 controller test updated. Manifest
declares xlsxwriter as an external_dependency.

Made-with: Cursor

fusion_accounting_reports/tests/test_reports_controller.py

@@ -111,10 +111,16 @@ class TestReportsController(HttpCase):
         self.assertIn('pdf_base64', result)
         self.assertTrue(result.get('filename', '').endswith('.pdf'))
 
-    def test_export_xlsx_placeholder(self):
+    def test_export_xlsx_returns_xlsx(self):
+        try:
+            import xlsxwriter  # noqa: F401
+        except ImportError:
+            self.skipTest("xlsxwriter not installed")
         result = self._jsonrpc('export_xlsx', {
             'report_type': 'pnl',
             'date_from': '2026-01-01',
             'date_to': '2026-12-31',
         })
-        self.assertEqual(result.get('status'), 'not_implemented')
+        self.assertEqual(result.get('status'), 'ok')
+        self.assertTrue(result.get('xlsx_base64'))
+        self.assertTrue(result.get('filename', '').endswith('.xlsx'))