Feature: add ability to unmap receipts from transactions

Add unmap functionality to allow users to disassociate receipts from transactions without deleting them:

- ReceiptManager: Add UnmapReceiptAsync method
- Receipts page: Add OnPostUnmapAsync handler
- Receipts view: Add Unmap button for mapped receipts with confirmation dialog

This provides a non-destructive alternative to deleting receipts when they need to be remapped to different transactions.

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

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

MoneyMap/Pages/Receipts.cshtml

@@ -235,6 +235,14 @@
                                                 Map
                                             </button>
                                         }
+                                        else
+                                        {
+                                            <form method="post" asp-page-handler="Unmap" asp-route-receiptId="@r.Id" style="display: inline;" onsubmit="return confirm('Are you sure you want to unmap this receipt from the transaction?');">
+                                                <button type="submit" class="btn btn-outline-warning btn-sm" title="Unmap from Transaction">
+                                                    Unmap
+                                                </button>
+                                            </form>
+                                        }
                                         <form method="post" asp-page-handler="Delete" asp-route-receiptId="@r.Id" style="display: inline;" onsubmit="return confirm('Are you sure you want to delete this receipt?');">
                                             <button type="submit" class="btn btn-outline-danger btn-sm" title="Delete">
                                                 Delete

MoneyMap/Pages/Receipts.cshtml.cs

@@ -218,6 +218,24 @@ namespace MoneyMap.Pages
             return RedirectToPage();
         }
 
+        public async Task<IActionResult> OnPostUnmapAsync(long receiptId)
+        {
+            var success = await _receiptManager.UnmapReceiptAsync(receiptId);
+
+            if (success)
+            {
+                Message = "Receipt unmapped successfully.";
+                IsSuccess = true;
+            }
+            else
+            {
+                Message = "Failed to unmap receipt.";
+                IsSuccess = false;
+            }
+
+            return RedirectToPage();
+        }
+
         private async Task LoadReceiptsAsync()
         {
             var receipts = await _db.Receipts

MoneyMap/Services/ReceiptManager.cs

@@ -12,6 +12,7 @@ namespace MoneyMap.Services
         Task<ReceiptUploadResult> UploadUnmappedReceiptAsync(IFormFile file);
         Task<bool> DeleteReceiptAsync(long receiptId);
         Task<bool> MapReceiptToTransactionAsync(long receiptId, long transactionId);
+        Task<bool> UnmapReceiptAsync(long receiptId);
         string GetReceiptPhysicalPath(Receipt receipt);
         Task<Receipt?> GetReceiptAsync(long receiptId);
     }
@@ -210,6 +211,19 @@ namespace MoneyMap.Services
             return true;
         }
 
+        public async Task<bool> UnmapReceiptAsync(long receiptId)
+        {
+            var receipt = await _db.Receipts.FindAsync(receiptId);
+            if (receipt == null)
+                return false;
+
+            // Set TransactionId to null to unmap
+            receipt.TransactionId = null;
+            await _db.SaveChangesAsync();
+
+            return true;
+        }
+
         private static string SanitizeFileName(string fileName)
         {
             if (string.IsNullOrWhiteSpace(fileName))
@@ -219,7 +233,7 @@ namespace MoneyMap.Services
             var sanitized = new StringBuilder();
             foreach (var c in fileName)
             {
-                if (c == '®' || c == '™' || c == '©')
+                if (c == '<EFBFBD>' || c == '<EFBFBD>' || c == '<EFBFBD>')
                 {
                     // Skip trademark/copyright symbols
                     continue;