Add JSON export/import functionality for category mappings

Add the ability to export and import category mappings as JSON files:

Export:
- Downloads all category mappings as a formatted JSON file
- Ordered by category and priority

Import:
- Upload JSON file with category mappings
- Option to replace all existing mappings or merge with current ones
- Validates JSON format and data integrity
- Shows clear error messages for invalid files

UI additions:
- Export to JSON button in header
- Import from JSON button with modal dialog
- Shows example JSON format in import modal
- Replace/merge option with clear explanation

This allows users to:
- Backup their custom category mappings
- Share mappings between instances
- Version control their categorization rules
- Easily restore default or custom configurations

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

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

MoneyMap/Pages/CategoryMappings.cshtml

@@ -6,7 +6,13 @@
 
 <div class="d-flex justify-content-between align-items-center mb-3">
     <h2>Category Mappings</h2>
-    <div>
+    <div class="d-flex gap-2">
+        <a asp-page-handler="Export" class="btn btn-outline-primary">
+            <span>⬇</span> Export to JSON
+        </a>
+        <button type="button" class="btn btn-outline-success" data-bs-toggle="modal" data-bs-target="#importModal">
+            <span>⬆</span> Import from JSON
+        </button>
         <a asp-page="/Index" class="btn btn-outline-secondary">Back to Dashboard</a>
     </div>
 </div>
@@ -227,6 +233,60 @@ else
     </div>
 </div>
 
+<!-- Import Modal -->
+<div class="modal fade" id="importModal" tabindex="-1" aria-labelledby="importModalLabel" aria-hidden="true">
+    <div class="modal-dialog">
+        <div class="modal-content">
+            <form method="post" asp-page-handler="Import" enctype="multipart/form-data">
+                <div class="modal-header">
+                    <h5 class="modal-title" id="importModalLabel">Import Categories from JSON</h5>
+                    <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
+                </div>
+                <div class="modal-body">
+                    <div class="mb-3">
+                        <label for="jsonFile" class="form-label">Select JSON File</label>
+                        <input type="file" name="jsonFile" id="jsonFile" class="form-control" accept=".json" required />
+                        <div class="form-text">Upload a JSON file with category mappings</div>
+                    </div>
+
+                    <div class="mb-3">
+                        <div class="form-check">
+                            <input class="form-check-input" type="checkbox" name="replaceExisting" id="replaceExisting" value="true" />
+                            <label class="form-check-label" for="replaceExisting">
+                                <strong>Replace all existing mappings</strong>
+                            </label>
+                            <div class="form-text">
+                                If checked, all current mappings will be deleted and replaced with the imported ones.
+                                If unchecked, imported mappings will be added to existing ones.
+                            </div>
+                        </div>
+                    </div>
+
+                    <div class="alert alert-info mb-0">
+                        <strong>JSON Format:</strong>
+                        <pre class="mb-0 mt-2 small">[
+  {
+    "category": "Groceries",
+    "pattern": "WALMART",
+    "priority": 0
+  },
+  {
+    "category": "Gas & Auto",
+    "pattern": "SHELL",
+    "priority": 100
+  }
+]</pre>
+                    </div>
+                </div>
+                <div class="modal-footer">
+                    <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Cancel</button>
+                    <button type="submit" class="btn btn-success">Import</button>
+                </div>
+            </form>
+        </div>
+    </div>
+</div>
+
 @section Scripts {
     <script>
         function openEditModal(id, category, pattern, priority) {

MoneyMap/Pages/CategoryMappings.cshtml.cs

@@ -1,7 +1,10 @@
 using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.Linq;
+using System.Text;
+using System.Text.Json;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.RazorPages;
 using Microsoft.EntityFrameworkCore;
@@ -108,6 +111,97 @@ namespace MoneyMap.Pages
             return RedirectToPage();
         }
 
+        public async Task<IActionResult> OnGetExportAsync()
+        {
+            var mappings = await _db.CategoryMappings
+                .OrderBy(m => m.Category)
+                .ThenByDescending(m => m.Priority)
+                .ToListAsync();
+
+            var exportData = mappings.Select(m => new CategoryMappingExport
+            {
+                Category = m.Category,
+                Pattern = m.Pattern,
+                Priority = m.Priority
+            }).ToList();
+
+            var json = JsonSerializer.Serialize(exportData, new JsonSerializerOptions
+            {
+                WriteIndented = true
+            });
+
+            var bytes = Encoding.UTF8.GetBytes(json);
+            return File(bytes, "application/json", "category-mappings.json");
+        }
+
+        public async Task<IActionResult> OnPostImportAsync(IFormFile jsonFile, bool replaceExisting)
+        {
+            if (jsonFile == null || jsonFile.Length == 0)
+            {
+                ErrorMessage = "Please select a JSON file to import.";
+                return RedirectToPage();
+            }
+
+            try
+            {
+                using var stream = jsonFile.OpenReadStream();
+                var importData = await JsonSerializer.DeserializeAsync<List<CategoryMappingExport>>(stream, new JsonSerializerOptions
+                {
+                    PropertyNameCaseInsensitive = true
+                });
+
+                if (importData == null || !importData.Any())
+                {
+                    ErrorMessage = "No category mappings found in the file.";
+                    return RedirectToPage();
+                }
+
+                // Validate the data
+                foreach (var item in importData)
+                {
+                    if (string.IsNullOrWhiteSpace(item.Category) || string.IsNullOrWhiteSpace(item.Pattern))
+                    {
+                        ErrorMessage = "Invalid data in JSON file. Each mapping must have a category and pattern.";
+                        return RedirectToPage();
+                    }
+                }
+
+                // Replace existing or merge
+                if (replaceExisting)
+                {
+                    var existingMappings = await _db.CategoryMappings.ToListAsync();
+                    _db.CategoryMappings.RemoveRange(existingMappings);
+                }
+
+                // Add new mappings
+                var newMappings = importData.Select(m => new CategoryMapping
+                {
+                    Category = m.Category.Trim(),
+                    Pattern = m.Pattern.Trim(),
+                    Priority = m.Priority
+                }).ToList();
+
+                _db.CategoryMappings.AddRange(newMappings);
+                await _db.SaveChangesAsync();
+
+                SuccessMessage = replaceExisting
+                    ? $"Successfully replaced all mappings with {importData.Count} mappings from file."
+                    : $"Successfully imported {importData.Count} mappings from file.";
+
+                return RedirectToPage();
+            }
+            catch (JsonException)
+            {
+                ErrorMessage = "Invalid JSON file format. Please check the file and try again.";
+                return RedirectToPage();
+            }
+            catch (Exception ex)
+            {
+                ErrorMessage = $"Error importing file: {ex.Message}";
+                return RedirectToPage();
+            }
+        }
+
         private async Task LoadDataAsync()
         {
             var mappings = await _categorizer.GetAllMappingsAsync();
@@ -162,5 +256,12 @@ namespace MoneyMap.Pages
 
             public int Priority { get; set; } = 0;
         }
+
+        public class CategoryMappingExport
+        {
+            public string Category { get; set; } = "";
+            public string Pattern { get; set; } = "";
+            public int Priority { get; set; } = 0;
+        }
     }
 }