Refactor: externalize OpenAI receipt parser prompt to text file

Move hardcoded prompt from OpenAIReceiptParser.cs to Prompts/ReceiptParserPrompt.txt for easier maintenance and editing without recompilation. Add caching to LoadPromptTemplateAsync for performance. Configure .csproj to copy Prompts folder to output directory.

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

MoneyMap/Prompts/ReceiptParserPrompt.txt

@@ -0,0 +1,28 @@
+Analyze this receipt image and extract the following information as JSON:
+{
+  "merchant": "store name",
+  "receiptDate": "YYYY-MM-DD" (or null if not found),
+  "dueDate": "YYYY-MM-DD" (or null if not found - for bills only),
+  "subtotal": 0.00 (or null if not found),
+  "tax": 0.00 (or null if not found),
+  "total": 0.00,
+  "confidence": 0.95,
+  "lineItems": [
+    {
+      "description": "item name",
+      "quantity": 1.0 (or null),
+      "unitPrice": 0.00 (or null),
+      "lineTotal": 0.00
+    }
+  ]
+}
+
+Extract all line items you can see on the receipt. For each item:
+- description: The item or service name
+- quantity: Only include if this is an actual countable product (like groceries). For services, fees, charges, or taxes, set to null.
+- unitPrice: Price per unit if quantity applies, otherwise null
+- lineTotal: The total amount for this line (required)
+
+For utility bills, service charges, fees, and taxes - these are NOT products with quantities, so set quantity and unitPrice to null.
+
+If this is a bill (utility, credit card, etc.), look for a due date, payment due date, or deadline and extract it as dueDate. For regular receipts, dueDate should be null.