feat: P2 strankovani - PaginationHelper, Pagination komponenta, integrace do 4 modulu

- PaginationHelper.php: parseParams() + paginate() - DRY backend pagination logika
- Pagination.jsx: frontend strankovaci komponenta (prev/next/cisla/info)
- CSS: .admin-pagination styly v admin.css
- Refaktor handleru: offers, orders, invoices, projects pouzivaji PaginationHelper
- Default 25 zaznamu na stranku (misto 500), max 500
- Frontend: page state + reset na search/filter zmenu
- useListData: pagination data v response

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/admin/handlers/invoices-handlers.php

@@ -169,12 +169,7 @@ function handleGetStats(PDO $pdo): void
 
 function handleGetList(PDO $pdo): void
 {
-    $search = trim($_GET['search'] ?? '');
     $statusFilter = trim($_GET['status'] ?? '');
-    $sort = $_GET['sort'] ?? 'created_at';
-    $order = strtoupper($_GET['order'] ?? 'DESC') === 'ASC' ? 'ASC' : 'DESC';
-    $page = max(1, (int) ($_GET['page'] ?? 1));
-    $perPage = min(500, max(1, (int) ($_GET['per_page'] ?? 500)));
 
     $sortMap = [
         'InvoiceNumber' => 'i.invoice_number',
@@ -188,10 +183,8 @@ function handleGetList(PDO $pdo): void
         'IssueDate' => 'i.issue_date',
         'issue_date' => 'i.issue_date',
     ];
-    if (!isset($sortMap[$sort])) {
-        errorResponse('Neplatný parametr řazení', 400);
-    }
-    $sortCol = $sortMap[$sort];
+
+    $p = PaginationHelper::parseParams($sortMap);
 
     // Lazy overdue detekce
     $pdo->exec("UPDATE invoices SET status = 'overdue' WHERE status = 'issued' AND due_date < CURDATE()");
@@ -199,10 +192,9 @@ function handleGetList(PDO $pdo): void
     $where = 'WHERE 1=1';
     $params = [];
 
-    if ($search) {
-        $search = mb_substr($search, 0, 100);
+    if ($p['search']) {
         $where .= ' AND (i.invoice_number LIKE ? OR c.name LIKE ? OR c.company_id LIKE ?)';
-        $searchParam = "%{$search}%";
+        $searchParam = "%{$p['search']}%";
         $params = array_merge($params, [$searchParam, $searchParam, $searchParam]);
     }
 
@@ -215,36 +207,26 @@ function handleGetList(PDO $pdo): void
         }
     }
 
-    $countSql = "
-        SELECT COUNT(*)
-        FROM invoices i
+    $from = "FROM invoices i
         LEFT JOIN customers c ON i.customer_id = c.id
-        $where
-    ";
-    $stmt = $pdo->prepare($countSql);
-    $stmt->execute($params);
-    $total = (int) $stmt->fetchColumn();
+        LEFT JOIN orders o ON i.order_id = o.id";
 
-    $offset = ($page - 1) * $perPage;
-
-    $sql = "
-        SELECT i.id, i.invoice_number, i.order_id, i.status, i.currency,
+    $result = PaginationHelper::paginate(
+        $pdo,
+        "SELECT COUNT(*) FROM invoices i LEFT JOIN customers c ON i.customer_id = c.id {$where}",
+        "SELECT i.id, i.invoice_number, i.order_id, i.status, i.currency,
                i.issue_date, i.due_date, i.paid_date, i.created_at, i.apply_vat,
                c.name as customer_name,
                (SELECT COALESCE(SUM(ii.quantity * ii.unit_price), 0)
                 FROM invoice_items ii WHERE ii.invoice_id = i.id) as subtotal,
                o.order_number
-        FROM invoices i
-        LEFT JOIN customers c ON i.customer_id = c.id
-        LEFT JOIN orders o ON i.order_id = o.id
-        $where
-        ORDER BY $sortCol $order
-        LIMIT $perPage OFFSET $offset
-    ";
+        {$from} {$where}
+        ORDER BY {$p['sort']} {$p['order']}",
+        $params,
+        $p
+    );
 
-    $stmt = $pdo->prepare($sql);
-    $stmt->execute($params);
-    $invoices = $stmt->fetchAll();
+    $invoices = $result['items'];
 
     // Dopocitat celkovou castku s DPH
     foreach ($invoices as &$inv) {
@@ -265,9 +247,7 @@ function handleGetList(PDO $pdo): void
 
     successResponse([
         'invoices' => $invoices,
-        'total' => $total,
-        'page' => $page,
-        'per_page' => $perPage,
+        'pagination' => $result['pagination'],
     ]);
 }