Feature: add Unmap button to prevent accidental receipt deletion

Add safer workflow for fixing incorrectly mapped receipts:
- Add OnPostUnmapReceiptAsync handler that sets TransactionId to null
- Add "Unmap" button (warning color) before Delete button
- Update Delete confirmation to emphasize PERMANENT deletion
- Unmap success message guides user to Receipts page

This prevents accidental loss of receipt files when users just want to remap them to a different transaction.

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

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

MoneyMap/Pages/EditTransaction.cshtml

@@ -159,7 +159,7 @@
                                         </div>
                                         <small class="text-muted">
                                             @((item.Receipt.FileSizeBytes / 1024.0).ToString("F1")) KB
-                                            · @item.Receipt.UploadedAtUtc.ToLocalTime().ToString("MMM d, yyyy")
+                                            <EFBFBD> @item.Receipt.UploadedAtUtc.ToLocalTime().ToString("MMM d, yyyy")
                                         </small>
                                         @if (!string.IsNullOrWhiteSpace(item.Receipt.Merchant))
                                         {
@@ -208,8 +208,11 @@
                                             <button type="submit" class="btn btn-sm btn-outline-info">Parse</button>
                                         </form>
                                     }
+                                    <form method="post" asp-page-handler="UnmapReceipt" asp-route-receiptId="@item.Receipt.Id" class="d-inline">
+                                        <button type="submit" class="btn btn-sm btn-outline-warning" title="Unmap from this transaction (receipt will go back to Receipts page)">Unmap</button>
+                                    </form>
                                     <form method="post" asp-page-handler="DeleteReceipt" asp-route-receiptId="@item.Receipt.Id"
-                                          onsubmit="return confirm('Delete this receipt?')" class="d-inline">
+                                          onsubmit="return confirm('Are you sure? This will PERMANENTLY DELETE the receipt file!')" class="d-inline">
                                         <button type="submit" class="btn btn-sm btn-outline-danger">Delete</button>
                                     </form>
                                 </div>

MoneyMap/Pages/EditTransaction.cshtml.cs

@@ -181,13 +181,30 @@ namespace MoneyMap.Pages
             return RedirectToPage(new { id = Transaction.Id });
         }
 
+        public async Task<IActionResult> OnPostUnmapReceiptAsync(long receiptId)
+        {
+            var receipt = await _db.Receipts.FindAsync(receiptId);
+            if (receipt != null)
+            {
+                receipt.TransactionId = null;
+                await _db.SaveChangesAsync();
+                SuccessMessage = "Receipt unmapped successfully! You can now find it on the Receipts page.";
+            }
+            else
+            {
+                ErrorMessage = "Receipt not found.";
+            }
+
+            return RedirectToPage(new { id = Transaction.Id });
+        }
+
         public async Task<IActionResult> OnPostDeleteReceiptAsync(long receiptId)
         {
             var success = await _receiptManager.DeleteReceiptAsync(receiptId);
 
             if (success)
             {
-                SuccessMessage = "Receipt deleted successfully!";
+                SuccessMessage = "Receipt deleted permanently!";
             }
             else
             {