Add due date support for bills and utility receipts

Enhanced receipt parsing and mapping to support bills with due dates (like utility bills):
- Updated OpenAI prompt to extract due date from bills
- Added DueDate property to Receipt model with database migration
- Modified auto-mapping logic to use bill date to due date range when both dates are present
- For regular receipts without due dates, continues to use ±3 day window
- Updated manual mapping UI to display due dates and adjust date range explanation
- Receipt list now shows due dates in orange "Due:" text for clarity

This allows electric bills and similar documents with 20+ day payment windows to be properly matched to their payment transactions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.claude/settings.local.json

@@ -2,7 +2,9 @@
   "permissions": {
     "allow": [
       "Bash(git add:*)",
-      "Bash(git commit:*)"
+      "Bash(git commit:*)",
+      "Bash(dotnet ef migrations:*)",
+      "Bash(dotnet ef database:*)"
     ],
     "deny": [],
     "ask": []