Add pagination to transaction preview for large imports

When uploading files with more than 100 transactions, the preview now displays transactions in pages of 100 rows at a time. This prevents browser freezing when rendering thousands of form inputs and dramatically improves page load performance.

- Show first 100 transactions by default with pagination controls
- All transactions remain in DOM but hidden for instant page switching
- Update counters and form submission to work across all pages

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

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

MoneyMap/Pages/Upload.cshtml

@@ -44,6 +44,14 @@
         <div class="card shadow-sm mb-3">
             <div class="card-header d-flex justify-content-between align-items-center">
                 <strong>Transaction Preview (<span id="selectedCount">@Model.PreviewTransactions.Count(p => !p.IsDuplicate)</span> selected, @Model.PreviewTransactions.Count(p => p.IsDuplicate) duplicates)</strong>
+                @if (Model.PreviewTransactions.Count > 100)
+                {
+                    <div class="d-flex align-items-center gap-2">
+                        <small class="text-muted">Showing <span id="pageStart">1</span>-<span id="pageEnd">100</span> of @Model.PreviewTransactions.Count</small>
+                        <button type="button" class="btn btn-sm btn-outline-secondary" onclick="previousPage()">Previous</button>
+                        <button type="button" class="btn btn-sm btn-outline-secondary" onclick="nextPage()">Next</button>
+                    </div>
+                }
             </div>
             <div class="card-body p-0">
                 <div class="table-responsive" style="max-height: 500px; overflow-y: auto;">
@@ -62,11 +70,12 @@
                                 <th style="width: 80px;">Status</th>
                             </tr>
                         </thead>
-                        <tbody>
+                        <tbody id="transactionTableBody">
                             @for (int i = 0; i < Model.PreviewTransactions.Count; i++)
                             {
                                 var preview = Model.PreviewTransactions[i];
-                                <tr class="@(preview.IsDuplicate ? "table-secondary text-muted" : "")" data-index="@i">
+                                var displayStyle = i >= 100 ? "display: none;" : "";
+                                <tr class="@(preview.IsDuplicate ? "table-secondary text-muted" : "") transaction-row" data-index="@i" style="@displayStyle">
                                     <td>
                                         <input type="checkbox" data-index="@i"
                                                class="form-check-input transaction-checkbox"
@@ -227,6 +236,10 @@ else
 @section Scripts {
     <partial name="_ValidationScriptsPartial" />
     <script>
+        let currentPage = 0;
+        const pageSize = 100;
+        const totalRows = @Model.PreviewTransactions.Count;
+
         function togglePaymentSelection() {
             const mode = document.querySelector('input[name="PaymentMode"]:checked').value;
             const cardRow = document.getElementById('cardSelectRow');
@@ -246,8 +259,10 @@ else
         function updateSelectedCount() {
             const checkboxes = document.querySelectorAll('.transaction-checkbox:checked');
             const count = checkboxes.length;
-            document.getElementById('selectedCount').textContent = count;
+            const selectedCountEl = document.getElementById('selectedCount');
-            document.getElementById('selectedCountButton').textContent = count;
+            const selectedCountButtonEl = document.getElementById('selectedCountButton');
+            if (selectedCountEl) selectedCountEl.textContent = count;
+            if (selectedCountButtonEl) selectedCountButtonEl.textContent = count;
         }
 
         function updateAllTransactionAccounts() {
@@ -255,6 +270,39 @@ else
             // No action needed - we'll read the global account on form submit
         }
 
+        function showPage(pageNum) {
+            const rows = document.querySelectorAll('.transaction-row');
+            const start = pageNum * pageSize;
+            const end = start + pageSize;
+
+            rows.forEach((row, index) => {
+                row.style.display = (index >= start && index < end) ? '' : 'none';
+            });
+
+            // Update pagination display
+            const pageStartEl = document.getElementById('pageStart');
+            const pageEndEl = document.getElementById('pageEnd');
+            if (pageStartEl && pageEndEl) {
+                pageStartEl.textContent = start + 1;
+                pageEndEl.textContent = Math.min(end, totalRows);
+            }
+
+            currentPage = pageNum;
+        }
+
+        function nextPage() {
+            const maxPage = Math.ceil(totalRows / pageSize) - 1;
+            if (currentPage < maxPage) {
+                showPage(currentPage + 1);
+            }
+        }
+
+        function previousPage() {
+            if (currentPage > 0) {
+                showPage(currentPage - 1);
+            }
+        }
+
         function prepareFormData() {
             // Collect selected indices
             const selectedIndices = [];
@@ -264,7 +312,7 @@ else
             document.getElementById('selectedIndices').value = selectedIndices.join(',');
 
             // Get the global account ID
-            const globalAccountId = document.getElementById('globalAccountId').value;
+            const globalAccountId = document.getElementById('globalAccountId')?.value;
             if (!globalAccountId) {
                 alert('Please select an account for all transactions');
                 return false;
@@ -292,6 +340,11 @@ else
         }
 
         // Initialize on page load
-        document.addEventListener('DOMContentLoaded', togglePaymentSelection);
+        document.addEventListener('DOMContentLoaded', function() {
+            togglePaymentSelection();
+            if (totalRows > pageSize) {
+                showPage(0);
+            }
+        });
     </script>
 }