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_xlsx_export.py

@@ -0,0 +1,36 @@
+"""Tests for XLSX export wizard."""
+
+from datetime import date
+from odoo.tests.common import TransactionCase, tagged
+
+
+@tagged('post_install', '-at_install')
+class TestXlsxExport(TransactionCase):
+
+    def test_export_pnl_produces_xlsx(self):
+        try:
+            import xlsxwriter  # noqa: F401
+        except ImportError:
+            self.skipTest("xlsxwriter not installed")
+        wizard = self.env['fusion.xlsx.export.wizard'].create({
+            'report_type': 'pnl',
+            'date_from': date(2026, 1, 1),
+            'date_to': date(2026, 12, 31),
+        })
+        wizard.action_export()
+        self.assertEqual(wizard.state, 'done')
+        self.assertTrue(wizard.xlsx_file)
+        self.assertTrue(wizard.xlsx_filename.endswith('.xlsx'))
+
+    def test_export_balance_sheet(self):
+        try:
+            import xlsxwriter  # noqa: F401
+        except ImportError:
+            self.skipTest("xlsxwriter not installed")
+        wizard = self.env['fusion.xlsx.export.wizard'].create({
+            'report_type': 'balance_sheet',
+            'date_from': date(2026, 1, 1),
+            'date_to': date(2026, 12, 31),
+        })
+        wizard.action_export()
+        self.assertEqual(wizard.state, 'done')