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/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting Reports',
-    'version': '19.0.1.0.30',
+    'version': '19.0.1.0.31',
     'category': 'Accounting/Accounting',
     'summary': 'AI-augmented financial reports (P&L, balance sheet, trial balance, GL).',
     'description': """
@@ -37,7 +37,11 @@ menu hides; the engine and AI tools remain available for the chat.
         'data/report_general_ledger.xml',
         'data/cron.xml',
         'reports/report_pdf_template.xml',
+        'wizards/xlsx_export_wizard_views.xml',
     ],
+    'external_dependencies': {
+        'python': ['xlsxwriter'],
+    },
     'assets': {
         'web.assets_backend': [
             'fusion_accounting_reports/static/src/scss/_variables.scss',