Add functionality to export payment schedules to Excel

Introduces an API endpoint and frontend components for exporting payment schedule data to an Excel file, with filtering options for payment methods and tax inclusion.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 1e4433b5-47ab-464c-b663-fea2e53367dd
Replit-Commit-Checkpoint-Type: intermediate_checkpoint
Replit-Commit-Event-Id: 09286923-d43e-46ea-831c-611649677d67
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/1e4433b5-47ab-464c-b663-fea2e53367dd/vfXE6SO

package.json

@@ -88,6 +88,7 @@
     "vaul": "^1.1.2",
     "wouter": "^3.3.5",
     "ws": "^8.18.0",
+    "xlsx": "^0.18.5",
     "zod": "^3.24.2",
     "zod-validation-error": "^3.4.0"
   },